What is a Nebula? – Updated

A nebula is a cloud of dust and gas found within the interstellar medium filling the great voids between the stars within galaxies and star clusters.

The different types of nebula consist of different elements in different proportions. Most nebulae that have not been formed by the destruction of dying stars (i.e. SNR (several types -Ia and II – will be described at a later date), planetary nebulae and those generated by Wolf-Rayet stars), contain large amounts of hydrogen gas. These nebulae if given the right conditions to compress and heat up will form the next generation of stars.

One such star forming nebula - The Eagle Nebula Credit NASA, ESA; HST

All stars, whether they are hypergiants or red dwarfs began their lives as a nebula and rather fittingly as a star dies it returns its material to the cosmos as another nebula. This nebula is either a planetary nebula or a supernova remnant, and it is through this release of matter that the universe is provided with all the elements heavier than hydrogen and helium. This includes all the material that forms the Earth and everything on it, including humans. The oxygen we breathe was formed in the hearts of red giants and the iron in our bloodstream was produced in the final days of a massive star’s existence, right before it ripped itself to pieces as a supernova. It is from this that we get the saying that we are all made of star dust, we quite literally are!

The Helix Planetary Nebula Credit NASA, ESA; HST Perhaps one day a new star will for from the ashes of the star that produced this lovely sight.

As each generation of stars further enriches the universe by spreading their life’s work as a nebula, the following generation of stars contain more of the heavier elements as there is now more available thanks to the previous generation synthesising (producing) them from hydrogen and helium over the course of their lives. Meaning that each successive stellar generation contains a larger quantity of ‘metals’ – in astrophysics a metal is any element other than hydrogen and helium – this allows different populations of stars to be identified based on their metal content. This variation is due to each successive generation of star forming nebulae contain more and more dust and metals hence creating the different detectable differences in the spectra of the stars they produce.

There are three main populations of stars though I shall keep a description of each for a further post more focused on the topic.

There are several very different types of nebulae but these types will be discussed in depth in further posts.

 

The MOST Humble Space Telescope

Space telescopes usually thought of as huge machines. The famous Hubble Space Telescope, for example, is 13.2 meters (43.5 ft) long and 4.2 meters (14 ft) in diameter. Though not all are so large; 800 kilometers above the Earth there is a satellite that is just 65 centimeters (about 25 inches) long – not larger than a large suitcase – proving that, when it comes to science, “size doesn’t matter”.

This astonishing device is called MOST (which stands for Microvariability and Oscillations of Stars). Although being very small compared to its peers, this satellite is helping scientists answer intriguing questions about stars, planets and even the Universe itself.

The MOST Space Telescope Credit: Canadian Space Agency

Staring at the Stars

Launched on June 30th 2003, MOST is the first space telescope to be entirely designed and built in Canada. As its name suggests, it is designed to take precise measurements of variations in intensity (the brightness) of stars in order to determinate their composition and age. The larger space telescopes cannot afford the time required for this task as to measure these oscillations, is necessary to keep the lens pointed at a single target in the sky for weeks at a time, and they can’t do so because of the high demand for their time.

Usually, astronomers use expensive ground-based telescopes to measure these stellar pulsations. However, this isn’t the best way to do so, since the readings are distorted by the Earth’s atmosphere. Moreover, the day-night cycle makes impossible to scientists to observe a star for 24 hours a day. Though with its orbit above the Earth’s atmosphere MOST can avoid both problems; and is able to look at any part of the sky continuously for up to seven weeks with a minimum of distortion.

The Secret life of Stars

The technology of this incredible telescope is helping astronomers figure out some very interesting things about stars, things well beyond our expectations. One of these discoveries was made as soon as the satellite became operational.

In 2004, the MOST team reported that Procyon (the brightest star in the constellation Canis Minor) shows no pulsations at all, contradicting more than 20 years of observations. Later, in 2006, the scientists realized that they were dealing with an unknown class of stars, the “slowly pulsating B supergiants”.

Furthermore, MOST has also been used to study exoplanets in alien star systems. Indeed, this is the only telescope – in space and on Earth – able to detect the light reflected by a planet orbiting around another star. Although not designed for this purpose, MOST is giving us a hint of what the atmosphere of those planets look like. It does this by detecting subtle variations in the light from either the planet or the star itself. MOST can see changes down to levels of one part to a million – or one ten thousandth of a percent!

“MOST has been very good at seeing things in the Universe that most people never expected or thought possible,” says Jaymie Matthews.

This post is part of the Young Astronomers’ Databank Project

Social Science – Science140

Last week saw the launch of a new science project on twitter:

The project is hoping to collect as many science based definitions, explanations and facts all in the form of tweets which contain no more than 140 characters.

No small feat!

The project is looking for as many people to contribute as possible on all topics of science right through from Astrophysics to Zoology. Best of all anyone can take part regardless of age or experience.

What is the goal of this project you ask?

We aim to collect these definitions, explanations and biographies over the next three months in the hopes of collating them in to a book – the proceeds of which will go to charity. Details of the charity / charities will be released in the coming weeks.

Fun education from all, for all and helping out a good cause to boot, what could be better!

You can learn more about the project on the official website and on twitter by following @science140

Submitting a tweet for the project is easy, just include the hash tag – #science140

So what are you waiting for! Dust off your facts and get tweeting!

A Star’s Death Giving Life to a Monster – Recovered

3.8 billion light years away in the constellation Draco deep inside the centre of an inconspicuous galaxy, something happened at 12:57:45 on the 28^th of March 2011 that flooded the SWIFT satellite’s sensors with x-rays, and in the process sent astronomers scrambling to get a glimpse with their ground and space-based observatories.

If you look at the light curve provided by SWIFT, the x-ray brightness fluctuates considerably over a period of days. You get the first massive burst, then it calms, and then you get some more bursts days after the original event. This is very different from GRBs, such events usually consist of a huge burst of x-rays and then a dimmer afterglow of a whole variety of radiation before fading from view over a period of hours at the most. So if it isn’t a GRB, then what is it?

The massive bursts happen to be coming from the centre of the galaxy, lighting up the heart of the galaxy with the power of 1 trillion suns; outshining the galaxy itself 100 times over. Like most of the galaxy population, a super massive black hole (SMBH) happens to lurk here. Could it be that the black hole has woken up? Active galaxies emit a huge amount of radiation, including X-rays, right across the electromagnetic spectrum after all.

With data from various surveys – including FERMI and ROSAT – astronomers have concluded that before this event there has been no  sign of activity from the SMBH for the past 20 years at least, so for it to flare up without warning is very  unusual!

So far the most popular theory with the most evidence suggests a main sequence star with a mass equivalent to our sun’s wandered too close to the gravitational grip of the SMBH; a monster weighing in at 10⁷ solar masses. During a single pass it would have had to put up with one side of it being stretched and tugged at more than the other, until the gravitational pull was so powerful that the star started to get torn apart.

The matter from the disintegrated star has now settled to form a temporary accretion disk that provides fodder for the black hole. The material in the disk started to interact, and a mixture of friction and magnetic fields collimated the radiation into jets which we view as head-on, drowning out the host galaxy with its luminosity.

If this is indeed the case, the bursts of radiation seen with SWIFT and other observatories should cease after a period of months to just over a year. This would show that the star is slowly getting devoured or spat out from the accretion disk, until one day there will be no fodder for the black hole at all, and it will settle back down into its dormant state and probably won’t wake up again until the galaxy merges, or another star falls prey to its gravity well.

There is another theory I’ve picked up from Arxiv by Dokuchaev et al. which is rather more exotic:

Instead of a star being destroyed via accretion, something massively destructive happened to a star cluster near the centre of the galaxy… But first, let me focus on GRBs.

There are two types of Gamma Ray Bursts; short GRBS and long GRBs. So, what’s the difference?

Long GRBs are the most common. They’re likely to come from Type 2 supernovae, the type of supernova you get when a high mass star implodes, leaving only a core or a black hole behind. The insanely bright jets of gamma rays are thought to come from the poles of stars that are collapsing into black holes and last up to a few minutes.

Short GRBs are less common, and are likely to come from the merger of two neutron stars or a neutron star colliding with a stellar mass black hole. They last less than two seconds, but are still as destructive to anything that lies in their path as the Long GRBS.

The star cluster mentioned earlier would have a whole variety of stars to choose from, including stellar remnants such as the ones mentioned above. The stars with the most mass will migrate to the centre of the cluster, until eventually the gravitational pull of each star in the vicinity causes them to interact with each other rather destructively…

Neutron stars start to collide with each other and stellar mass black holes, creating plenty of Short GRBs as they go along. The many GRBs account for the repeating flares recorded by SWIFT and other observatories. In the period of two days 7% of the stars that make up the cluster collapsed into an accreting super massive black hole!

If this theory is correct the black hole won’t shut down any time soon like in the most popular theory, but will carry on for many years as it gains in mass over time from devouring the remainder of the star cluster and perhaps beyond.

However, I’m standing by the first theory  Either way there’s some very interesting speculation surrounding this amazing object!

You can read more on this event here and here (both in PDF format) and on NASA.

WISE Shows the Sky is Awash with Blazars

The latest release from NASA’s WISE mission has shown that just over 200 previously unidentified high energy objects are likely to be blazars.

Artist's Impression of an active Blazar Credit:NASA/JPL-Caltech

A blazar is a form of active galactic nucleus (AGN) – a galaxy where the central black hole is ‘feeding’ on large amounts of material resulting in the release of huge amounts of radiation including two tight very bright jets.

The angle at which the AGN is situated relative to the Earth determines which form of AGN we observe even though all are tied to the same processes.

In the case of a blazar we are looking directly down the AGN’s jets you could even say right down the barrel of the gun!

AGN at various angles; Credit: Aurore Simonnet, SSU NASA E/PO.

As the AGN must be lined up almost exactly with Earth for a blazar to be observed they are understandably rare compared to the other forms of AGN which have a much larger range of possible viewing angles. That being said the WISE data has the potential to reveal several thousand more.

A team using the WISE data looked at 300 objects that had previously been detected as high energy gamma-ray sources by the Fermi Space Telescope, though up to now had remained unidentified.

Using WISE the team was able to observe these gamma ray hotspots in infra-red wavelengths and showed that just over half are most likely to be blazars. WISE had also observed 50 new blazars outside those Fermi oddities along with taking observations of more than 1000 previously identified blazar candidates.

One of the project leads, Francesco Massaro has explained that there may be several thousand more as of yet unknown blazars hidden within the WISE data that could be revealed using the techniques developed for this first sample.

An image of one of the new WISE identified blazars Credit:NASA/JPL-Caltech/Kavli

You can read more here

 

Origins of the Moon Challenged

The commonly accepted theory among scientists concerning Moon formation could be altered by a new study of lunar isotopes. (Variations of elements containing differing amounts of neutrons).

Luna Credit: NASA

Nearly half of the Moon was postulated to be from Theia, a planetary body, thought to have collided with Earth four point five billion years ago. The recent study by Junjun Zhang, an isotope geo-chemist at the Chicago Center for Cosmo-chemistry and his team challenged this premise. Initially, an analysis of twenty-four rocks from the lunar surface revealed a paucity of indications concerning similarities between the Moon and Earth. However, the group failed to consider the effect of cosmic rays, streams of charged particles racing through space. After amending their research on the Moon’s isotopes of titanium, they found its ratio to be in the ballpark of our planet’s chemistry.

The unlikelihood of Theia’s chemistry being nearly identical to the Earth prompted scientists to reconsider their model. They theorized, perhaps the planetary body collided and caused more joining of debris than previously suggested. This could infer, the majority of Theia’s constitution is hidden deep within the Moon, while Earth’s composition lays conspicuously on the surface.

Suggestions of a collision between a dual moon systems arose as well, inferring that one of our past satellites had a chemistry similar to the Earth. Scientists cannot know for sure, whether their ideas are correct or not, they are still theories. However, the research team plans on conducting more experiments on the isotopes of different elements found on the Moon.

You can read more about these findings at http://news.yahoo.com/moon-formation-theory-challenged-study-160608598.html

Astronomy Tech – Cassini-Huygens – Recovered

Since the beginning of the Space Age, man has sent many manned and unmanned missions into space. Very powerful telescopes, built around the world, broaden our vision and understanding of the universe. Spacecraft, whether visiting other worlds or orbiting the Earth send us images and data collected from our outer atmosphere to the outer planets and beyond.

However, all this was only possible thanks to the incredibly rapid development of technology in recent years. Only then, could the essential resources for the construction of the current generation of spaceships be developed.

So, let us talk a little about some of the most important of space exploration’s tools and its greatest discoveries in this series, called Astronomy Tech.

In this first post, let’s get to know Cassini-Huygens a bit better. It is a joint mission between NASA, the European Space Agency (ESA) and the Italian Space Agency (ISA), which has uncovering the secrets of Saturn, including its rings and moons, as its primary objective.

An artist's impression of Cassini-Huygens Credit: NASA

On October 15th, 1997, the Cassini-Huygens spacecraft – composed of NASA’s Cassini orbiter and the ESA’s Huygens probe – was launched, beginning a long and complex seven-year journey, including gravitational slingshot manoeuvres around Venus, Earth and Jupiter. After arriving at its destination, the mother ship; Cassini, began its main objective exploring Saturn, whilst the Huygens probe was lunched and landed on Titan –Saturn’s largest moon and the second largest in the Solar System, after Jupiter’s moon Ganymede.

The spacecraft’s name was a tribute to the Italian Jean-Dominique Cassini (1625-1712) – discover of the Saturnine satellites Iapetus, Rhea, Tethys and Dione. In 1675 he discovered what is known today as the ‘Cassini Division’, the narrow gap separating Saturn’s A and B rings. Christiaan Huygens (1629-1695) was a Dutch scientist who first described Saturn’s rings and, in 1655 he discovered the moon Titan.

Cassini’s “senses”

The Cassini spacecraft has a set of 12 instruments on-board. Some of them work in similar ways to our own. However, the instruments on the Cassini spacecraft are much more advanced than our own.

Cassini can “see” in wavelengths of light that the human eye cannot. The instruments on the spacecraft can “feel” things about magnetic fields and tiny dust particles that no human hand could detect. This means that Cassini can, for example ‘see the temperature’ of the objects it observes.

The magnetic field and particle detectors take direct sensing measurements of the environment around the spacecraft. These instruments measure magnetic fields, mass, electrical charges and densities of atomic particles. They also measure the quantity and composition of dust particles, the saturation of plasma (electrically charged gases), and radio waves.

Exploring the Ringed Planet

The expected return to Saturn – which hadn’t been visited by any spacecraft since Voyager 2 left Saturn’s orbit in 1981, – happened in July 2004. Since then, Cassini has made great discoveries about the Saturnine System and taken some terrific pictures, like the one below.

An Eclipse of Saturn, with the Rings Visible Credit: Cassini/NASA

A few days after reaching Saturn, Cassini released the Huygens probe to land on Titan. On January 14, 2005, during its fall, six instruments analysed Titan’s atmosphere. According to the returned data, Titan has a nitrogen rich atmosphere. It also confirmed that Titan’s orange colour is due to the presence of hydrocarbons, formed when sunlight breaks down the abundant methane molecules within the atmosphere.

These results have given scientists a glimpse of what Earth might have been like before life evolved. They now believe Titan possesses many similarities to the Earth, including lakes, rivers, channels, dunes, rain, snow, clouds, mountains and possibly volcanoes.

Cassini’s mission

Isn’t over yet; every day, it sends us vast amounts of data, back to astronomers allowing them to resolve and answer questions about Saturn and our own planet.

You can see more about Cassini on its official website. If you want to hear the news first hand, you can follow Cassini on Twitter.

Potential Research at the Poles

Note from the Young Astronomers Admin Team:

This is the first by our new editor YusefK and we would all like to welcome him to the team!

When you picture the world’s best site for astronomy, places like the Gemini Observatory in Chile or the W.M. Keck in Hawaii probably come to mind. Well think again, because the new epicenter of observing could be the coldest place on earth, Antarctica. After performing a careful analysis of the continent, scientists from America and Australia have pinpointed a prime location for ground based research. Several countries have already laid claim to the icy real estate, such as China, France, Russia and South Africa. The success of these countries’ scientific bases led to the development of more stations. We could owe our future understanding of the universe to those who work and live in this region.

Antarctica Credit: NASA, Davepape

Radio astronomers have already conducted research in the polar area. It was Martin Pomerantz who postulated that Antarctica was the best place for ground based astronomy. He was correct, but only under certain wavelengths. Until now, those who wanted to see the universe were out of luck. Nevertheless, the new site found by American and Australian scientists is expected to yield images three times sharper than today’s best observatories. Those of us who spend clear nights under the stars understand the importance of certain atmospheric factors. Things like water vapor in the air, temperature changes and darkness greatly affect our observing.

The location of this new site is a frozen plateau called Ridge A. Ridge A’s atmosphere is steady enough for average instruments to perform better than today’s common observatories. Imagine the potential of an eight inch Cassegrain in perfect weather conditions and the images would be stunning, when the larger facilities are established. The night sky above the summit is perfect for astronomical observing because it is calmer, dryer and darker than any ever known. Those who study and live in Antarctica will benefit from this treasure trove of unexplored skies. However, they will have to contend with several inhospitable factors foreign to the common scientist.

Living in Antarctica seems to be a challenge taken by the very eccentric or very passionate. Generally, scientists take a plane to the Falkland Islands and then a ship to Antarctica. There, they work with several professionals in the coldest and driest place on earth. Nevertheless, I have read the quarters are comfortable and the canned food isn’t half bad, either. Scientists work with a diverse group of people for months at a time and under strange and adverse conditions. So, for all budding astronomers and researchers, consider the South Pole for future endeavors in understanding the universe.

Sources:

http://www.antarctica.ac.uk/employment/locations/antarctica/living_in_antarctica.php

http://www.nsf.gov/pubs/1997/antpanel/4past.htm

http://www.physorg.com/news170932769.html

http://www.universetoday.com/38749/astronomers-find-worlds-best-observing-site/

The International Space Station

240 miles above your head a 420 tonne satellite orbits the Earth at 17000mph. It has been there, albeit in various states of construction, for 14 years, and for the last 11 of those it has been continuously occupied.

The fantastic photo taken by Italian ESA astronaut Paolo Nespoli from a Soyuz capsule that is currently my desktop background. Source: ESA/NASA

The International Space Station is a feat of engineering like no other. Not only does it demonstrate our technical ability to construct, launch, and maintain a permanent presence in space, but also our ability to coordinate the work of five different space agencies and their operations all over the planet.

But the journey from its conception has not been an easy one, the ISS was born out of three separate national programmes: NASA’s Freedom station, proposed in the early ‘80s as a response to the Soviet space stations Mir and Salyut, the Russian (formerly Soviet) Mir-2 project designed as a replacement for the aging Mir station, and the European Columbus space station project.

Budgetary constraints brought on by post-Cold War political changes made it increasingly clear that no single national programme was going to create a fully functioning scientific outpost. Instead the suggestion to combine the three programmes into a single international one was put forward and agreed in 1993 by US Vice-President Al Gore and Russian Prime Minister Viktor Chernomyrdin.

The first component, the Russian Zarya cargo block originally intended for the Mir-2 station was launched in 1998, and since then the station has expanded, first with the addition of connecting and services modules such as NASA’s Unity and RKA’s Zvezda, and later with more specialist modules such as ESA’s Columbus laboratory and the Cupola observation module, the largest window in space. In total the ISS consists of fifteen pressurised modules, with one more, Russian research laboratory Nauka still to be added. They comprise laboratories, docks and airlocks, and living areas, and their combined volume is just less than 1,000 cubic metres.

That all of these modules fitted together perfectly is a success story in itself. Many had not been built when the first pieces were launched, and for most their mating in orbit was the first time they were put together. Though there have been a few minor problems, they have always been resolved quickly, and at no point has the station ever had to be evacuated.

The station’s unique conditions have allowed a large variety of experiments to be performed, many of which would be impossible on Earth. Research is being done into how structures such as crystals and organic cells form and develop outside the influence of the Earth’s gravity. NASA is also taking the opportunity to do closer studies on the effects of prolonged exposure to microgravity on astronauts and the possible implications on future manned missions to the Moon, asteroids, or Mars.

Until the end of the shuttle program in August of last year, crew and supplies were transported by a variety of means including the space shuttles, and the Soyuz and Progress spacecraft. The 6-man Soyuz craft operated by Roscosmos, the Russian Federal Space Agency, is now the only method of sending new crews to the station.

Each contributory nation retains ownership of and responsibility for the components that it added. This responsibility extends to the disposal of the station when it reaches the end of its operational life, which the current time frame places somewhere in the 2020s, depending on whether and for how long its decommissioning is postponed after the initial 2020 date. Given the huge amount of money that has been invested in the station as well as the later than expected completion date, it is very likely that the ISS’s operational life will be extended some way beyond that deadline. By that point it is also expected that commercial space ventures will play a much larger role in the life and upkeep of the station, so they too may play some role in its end-of-life decisions.

Cosmic Candles – Updated

The universe is truly huge. It stretches for at least 13.5 billion light years (1.277×10²³ km) in every direction (considerably further in fact), and its massive size poses an equally large problem how can you accurately measure how far away objects are?

Over such large distances conventitional methods of measuring distances (like using the time taken for light to hit and object and reflect back) become impractical and very, very slow.

Thankfully astronomers can use other methods developed over the years to deal with these monumental universal distances. The three main methods they can achieve this are:

• Parallax
• Cepheid variables
• and Redshift

Parallax uses changes in the relative positions of celestial objects throughout the year to estimate how far away they are using trigonometry. A star that is closer to the Earth will appear to move more than a star that is further away. The same principle applies when you look out of the window in a moving car, objects close to you wizz past while those in the distance move lazily across your field of vision for some time (provided you don’t move your head of course).

Parallax is only useful for distances that are relatively small as after a certain point (16,000 light years distant – which may seem like a huge distance but it is actually less than 1% of our Galaxy’s diameter, tiny in comparison with the rest of the universe) the relative position of a distant object changes so little it is impossible to measure with any accuracy.

The standard unit of parallax is the Parsec or Parallax Arcsecond. Without going into too much detail of how it is calculated, a star that is one parsec away will have apparently changed in position by a fixed amount in the sky. A star that is two parsecs away will have appeared to have moved 1/2 as much and so on and so forth.

One parsec is approximately equal to 3.26 light years, which puts Earth’s nearest star (after the sun) Alpha Proxima Centauri at 1.29 parsecs away (4.2 light years).

Parallax forms the ‘bottom rung’ on the universal distance ladder (covering the shortest distances), skipping over the middle rung for now and moving to top – redshift.

Not only is the universe really, really big it is expanding – quickly. This is a direct result of the initial explosion that sparked our universe’s formation – the Big Bang. This was not an explosion in any conventional sense, it did not expand from a single point, instead it happened everywhere in the universe at once. This means that even today every single object in the universe is moving away from everything else (yes it is mind-blowing). This expansion is constant no matter where you are in the universe (as far as we can tell though this is the subject of some debate) and has been measured at 70.4 kms/Mpc (within a small range of uncertainty). Or in plain English, if two objects are a million parsecs away from each other they will be moving away from each other at 70.4 kilometres per second.

The Expanding Universe Credit: NASA / WMAP Science Team

Now at first glance this may not seem all that useful in determining distances, but over large distances it is a very powerful method indeed.

As this rate of expansion is fixed and is occurring throughout the universe, and most importantly everything is moving away from everything else (thanks to the space around it literally stretching), an object that is 2 million parsecs away will be moving at 140.8 kms, an object that is 3 million parsecs away will be whizzing away from us at 211.2 kms and so on.

That means we can infer the distance of an object by measuring its relative velocity. The only question is how do we measure the velocity of something that is literally half way across the universe? The answer comes from a somewhat unusual source.

Light as most people know travels at a fixed speed (3×10⁸ ms) in a vacuum. Regardless of what you try to do it, its speed is constant. As the universe expands you would expect light to appear to slow down as it has to travel further, in actual fact it stretches with the universe.

This manifests as in increase in the light’s wavelength (the distance between two adjacent, identical points on a wave). An increase in wavelength means the light has ‘lost’ energy and appearers redder hence the term redshift. Redshift is calculated by measuring the difference between observed spectral features in cellestial objects and comparing them to measurements taken here on Earth i.e. where redshift = 0. Redshift can be shortened to z and is the only practical method of determining distances to objects far into the universe.

So far we have dealt with Parallax which is used for very short distances (in universal standards), and Redshift which deals with very large distances. Whilst these are each very useful to have there is a reasonable gap between the two meaning that some objects within our own galaxy could not be distanced with any accuracy, and we would have no way of checking the reliability of either as there is practically no overlap in the measurement range of the two. This means that the results obtained for both methods could not be compared and checked to see if they agreed with each other.

Thankfully there is a middle rung on the cosmological distance lader – the Cepheid variables.

Cepheid variables (more accurately Population I Cepheids or Classical Cepheids) take their name from the second to be discovered - Delta Cephei.

Classical Cepheids are all yellow supergiant stars that fall in the spectral class bracket F6 – K2 (For information spectral classes of stars see here and here).

They were once main sequence B class stars like many of those found within the Pleiades open star cluster (right).

The PleiadesM45 Credit: Tad Denton/Adam Block/NOAO/AURA/NSF

These stars are all somewhere between 4 and 20 times the mass of the sun and spent just a few million years as a main sequence star fusing hydrogen into helium, before departing the main sequence and evolving into the supergiants we see today.

Such yellow supergiants are inherently unstable, having regular pulsations as the interior of the star changes in cycles which alter how much radiation the star retains causing it to swell and then contract as radiation is released into space at a faster rate causing it to return to its original size.

This cycle repeats again and again, in a relationship that is directly linked to the luminosity of the star. The more luminous the star the longer its pulsation period (the time taken for one expansion phase followed by one contraction phase). As this is fixed for all Cepheid variables regardless of their distance from Earth, two stars that have the same pulsation period with differing luminosities are at different distances.

That means that relative distances can be calculated by comparing Cepheids. Whilst that would be useful in itself there is more to the Cepheids, some of them are close enough to have their distance calculated by Parallax and other methods. So with a starting point for comparison the actual distances of all Cepheids regardless of their distance can be calculated accurately.

Cepheids have also been used to refine our results and estimates of important cosmological constants including values for the Hubble constant – a measure of how the universe is expanding.

So far around 700 classical Cepheids have been identified within the Milky Way, with an overall total of several thousand known as far out as NGC 4603 – 100 million light years away. Cepheids have been detected, and have had their properties measured, at distances at which redshift has become detectable bridging the gap between the two measurements and providing a way of comparing and confirming calculated distances for the very close and very distant universe.

For our mathematically minded readers (anyone else is welcome to have a look but if the site of an equation makes you cringe you are more than welcome to skip on ahead) I will now detail how to first calculate the luminosity of a classical Cepheid and then how to use the luminosity to calculate how far away the star is from Earth.

For the first stage all that is required is the pulsation period of the star. This is obtained by monitoring the star’s brightness over a long period of time and identifying the time for the star to dim from its peak brightness to its dimmest and to brighten back to its peak again. This is recorded for several cycles and averaged to reduce uncertainties in the measurements and to minimise the effect of any mild random fluctuations in the star itself.

The value for the period – P – is then substituted into the equation:

It looks rather complicated but the equation really isn’t, if you are careful it can even be done in one step using a calculator!

In this case M is the mean absolute magnitude of the star – a measure of distance adjusted brightness.

The second stage in the calculation is essentially a comparison between the mean absoultude magnitude and the mean apparent (average observed magnitude over the full pulsation cycle). Is this case the larger the difference the greater the distance involved – as of course, the further away an object is the dimmer it appears.

The equation we use in this case is:

Where:

• M is the calculated mean absolute magnitude
• m is the observed mean apparent magnitude
• and D is the distance in parsecs

This formula can in turn be rearranged to give an equation where D is in the subject:

And all you need do is plug in the numbers.

For example lets imagine we measure Cepheid to have a period of 51 days and a mean apparent magnitude of 18.5.

Its mean absolute magnitude would be:

-2.78(log51)-1.35 = M = -6.10 (Try it yourself if you don’t believe me!)

So now if we substitute M and m into the distance equation we get:

D = 10^4.92Parsecs

Which can also be written as: 83200 Parsecs or roughly 1.6 times the distance to the Large Magellanic Cloud.

There you go, not so scary after all!

An example of a Cepheid Variable (left) with its spectrum (below).

SDSS ID: 1237646647833788777 Credit: SDSS, DR8

Recently the distances calculated using Cepheids have been called into some doubt with concerns over the stars themselves.

It has long been though that their may be some change in the period of the star’s pulsations. This was predicted as stars slowly use mass over time as they blow out a fraction of their mass as their solar wind and in the emission of thermal and electromagnetic radiation (heat and light).

Spectrum of SDSS ID: 1237646647833788777 Credit: SDSS, DR8

This mass would then form a shell of material surrounding the star. This dust would absorb visible light from its parent star and re-emit it in the infra-red frequency range. Thus a star would appear dimmer than it truly is in visible light and brighter in infra-red light. Despite the differences being quite small it would introduce large uncertainties in the calculated distance of the star which in turn would cause serious inaccuracies in the values calculated from such measurements, thus proof of this mass loss and a way to compensate for it is vital in maintaining the integrity of the cosmic distance ladder and cosmology as a whole.

Recently, astrophysicists using NASA’s Sptizer Space Telescope have confirmed this mass loss by taking detailed images of the class’ namesake Delta Cephei. They show an intense bow shock surrounding the star, caused by the high speed interactions between the star’s stellar wind and the surrounding interstellar gas and dust.

The Bow Shock Surrounding Dela Cephei Credit: In image

Other observations also show similar bow shocks surrounding at least 25% of currently known Cepheids.

With these new measurements of mass loss a correction program can now be used to compensate for the reduction in luminosity of the variables and so allow for better measures of distances and more accurate calculation of the variables that define our universe.

You can read more on Cepheid Variables here