Date:  January 7, 2009

Title:  Understanding Variable Stars

Podcasters:  Travis Searle, Rebecca Turner, Mike Simonsen

Organization:  American Association of Variable Star Observers (AAVSO)

Description:   Variable stars are stars that change brightness. The brightness changes of these stars can range from a thousandth of a magnitude to as much as twenty magnitudes with periods ranging from seconds to years, depending on the type of variable star.  We’ll talk about the three main types of variable stars and what causes them to vary in brightness, as well as some of the more famous variable stars and how they got those strange lettered names.

Bio:   The AAVSO was founded in 1911 at Harvard College Observatory to coordinate variable star observations made largely by amateur astronomers. In 1954, the AAVSO became an independent, private research organization headquartered in Cambridge, Massachusetts. Today, with members in 45 countries and over 15.5 million observations, the AAVSO is the world’s largest association of variable star observers.  Membership in the AAVSO is open to anyone – professionals, amateurs, and educators alike – interested in variable stars and in contributing to the support of valuable research.

Today’s Sponsor:  Palomar Observatory

Transcript:

Restless Universe Show 1 – Understanding Variable Stars

Hello and welcome to ‘Restless Universe’, the podcast of the American Association of Variable Star Observers. Our focus is variable stars, and the fun amateur astronomers have while making significant contributions to science.

I’m your host Travis Searle and with me tonight are Rebecca Turner and Mike Simonsen. In Episode number 1, we are going to start at the beginning.

Okay, Mike. I guess we should first ask- What are variable stars?

Variable stars are stars that change brightness. These changes can take place in seconds or minutes; some may change in days or months. Others change over the course of years or decades.

Some of these changes are so small that it takes very precise instruments to detect them at all. Some stellar variation is so dramatic you can easily see it with the unaided eye, binoculars or through a backyard telescope.

So, even though the stars and constellations look permanent, peaceful and constant, there is a lot going on out there.

Definitely. It really is a restless universe.

Variable Star names:
Since we’ll be talking about variable stars, and mentioning some of them by name, this is probably a good time to talk about star names and the odd names we call these variable stars in particular.

Good idea. Many of the brightest stars have proper names. Some you may have heard before. Betelgeuse, Mira, Algol, Polaris. Others have names based on the Greek alphabet. Beta Lyrae, Gamma Cassieopia, Delta Cephei, Epsilon Aurigae, Eta Aquila, Chi Cygni, Delta Scorpii. By the way, all the stars we just named also happen to be variable stars!

Stars that don’t already have these proper or Greek letter names are named according to a system invented in the 19th century.

The first variable found in a constellation is given the letter R, the next S, and so on to the letter Z. The next star is named RR, then RS, and so on to RZ, SS to SZ, and so on to ZZ. Then the naming starts over at the beginning of the alphabet: AA, AB, and continuing on to QZ (the letter J is always omitted).

That’s crazy! Why did they make it so complicated?

When variable stars were first being discovered it was thought that this would be a rather rare phenomena, so they thought R through Z would be enough letters. Some stars already had names containing the letters A through Q, so they began with the upper case R to name variables. They kept extending the naming system to accommodate all the new discoveries until they ran out of double letter combinations.

This system can accommodate 334 names. But there are so many variables in some constellations that additional names are necessary. So after QZ, variables are simply named V335 (since 334 variables have already been named), V336, and so on.

So, the stars R, S, T and U Geminorum were the first variables discovered in Gemini. Usually the variables with single letters are fairly bright or they have large amplitudes, so they were the easiest to detect. The most recent variable discovered in Scorpio was named V1309 Scorpii, so it is the 1643rd variable discovered in that constellation!

Variability Types:
There are three main types of variable stars, pulsating, eruptive and eclipsing variables. Lets talk about pulsating variables first.

Pulsating variable star’s brightness variations are due the expansion and contraction of the surface layers of these stars. The star actually increases and decreases in size periodically. The different types of pulsating variables are distinguished by their periods of pulsation and the shapes of their light curves.

Pulsating variables are of great interest to astronomers. One of the most valuable properties of some of these pulsating stars is the direct relationship between the period of pulsation and their luminosity. This allows us to determine the distance to these stars, even if they are in far off distant galaxies.

Cepheids are very luminous, massive variables with periods of 1 -70 days. They are named after Delta Cephei, the first one discovered by John Goodricke in 1784. Classical Cepheids follow a well-defined period-luminosity relationship. The longer the period of the Cepheid, the more intrinsically luminous it is. This allows Cepheids to be used as ‘standard candles’ for distance determination. Edwin Hubble used Cepheids in the Andromeda galaxy to make the first estimate of the distance to another galaxy.

Next, there are the Long Period Variables, or LPVs as astronomers call them. Mira is probably the most famous, since it was the first pulsating variable ever discovered, in 1638.

That’s right. In fact, many LPVs are actually called Miras. The Mira-type stars have long periods, ranging from 80 to 1,000 days, and varying by 2.5 to 10 magnitudes visually. Mira itself has a period of 331 days and varies by almost 6 magnitudes. She is a red giant, with a radius that varies by about 20 percent. At the peak of expansion Mira is nearly 330 times the size of our Sun.

And there are other giant and supergiant, red pulsating stars called semi-regular variables.
As the name implies, these stars are less periodic in their light changes and can be somewhat unpredictable. They have periods ranging from a few days to several years and the change in brightness is typically less than two magnitudes. Betelgeuse and Antares are semi-regular variables you can see with the unaided eye.

Eruptive Variables:
Lets talk about my favorite variables, cataclysmic variables!
Eruptive variables can exhibit significant and rapid changes in brightness due to violent outbursts caused by processes within the star or the interaction of two stars orbiting each other very closely in space. First we’ll describe supernovae.

A supernova is a cataclysmic event towards the end of a star’s life resulting in a sudden, dramatic rise in brightness. A typical supernova may brighten by up to 20 magnitudes. A supernova can outshine the rest its galaxy for several days or a few weeks.

Supernovae are caused by one of two main mechanisms. The first is when accreting material falling onto a white dwarf in a binary system builds up until it reaches the Chandrasekhar limit, which is 1.4 times the mass of the sun. A white dwarf cannot exist as a white dwarf if it becomes more massive than this. Bad things happen. The resulting instability triggers an explosion that destroys the star and releases huge amounts of radioactive and heavy elements into space.

The second process occurs in very massive stars once all the material in the core has been fused into iron. Fusion can’t occur in elements heavier than iron. When the star evolves to this point, the outward radiation pressure drops. When that happens, the core rapidly implodes while the rest of the star is blown off into space. These types of supernovae may result in neutron stars and black holes.

And then there are novae. The word nova actually means ‘new star’. These stars were known to people long ago who thought they were literally new stars suddenly appearing in the sky.

We now know a nova is a close binary system rapidly rising in brightness by 7 – 16 magnitudes in just a few days. The eruptive event is followed by a steady decline back to the pre-outburst magnitude over a few months. Our model for novae is that of an accreting white dwarf, slowly drawing material off its close binary companion for 10,000 to 100,000 years, until there is sufficient material to trigger a thermonuclear explosion that blasts the shell of material off into space. Unlike a supernova, the nova event does not destroy the original star.
Recurrent Novae are similar to novae with outburst amplitudes of 7 – 16 magnitudes, lasting up to 200 days or so. But these novae have erupted two or more times in history.
Dwarf Novae are binary systems that exhibit a sudden increase in brightness by 2 to 5 magnitudes over a few days with intervals of weeks or months between outbursts.

As with other types of novae, dwarf novae are close binaries with a white dwarf as one of the component stars. The white dwarf is draining material off its close companion. This material can’t crash down right on to the surface of the white dwarf, so it goes into orbit around the star, creating what is called an accretion disk. The most popular model explaining their outbursts is the disk instability model in which thermal instabilities in the accretion disk cause outbursts but no catastrophic explosion.

So really, the main difference between novae, recurring novae and dwarf novae is the length of time between outbursts. Dwarf novae erupt often, the time between outbursts measured in days, months or years. Recurring novae may only go off once every few decades. Novae take so long to build up to an outburst that we only see them once in recorded history.

Eclipsing Variables:
Last, but not least are the eclipsing variables. These stars aren’t actually variable. It’s a line of sight effect that leads to fading at regular intervals.

Eclipsing binaries are pairs of stars in orbit around each other whose orbit just happens to line up with our line of sight to them. As one star passes in front of the other we observe an eclipse, just like when the moon passes in front of the sun from our point of view on earth, causing a solar eclipse. The light from the eclipsing binary fades and then brightens again as the eclipsing star moves in front of and then out of the way of the other star. The period of the eclipse, which coincides with the orbital period of the system, can range from minutes to years.

The study of variable stars permeates all branches of astronomy.  Supernovae are used to determine the scale of the universe; eclipsing binaries can reveal the mass, size and periods of common and exotic binary systems; pulsating stars yield clues about stellar interiors and evolution; accretion disks in cataclysmic variables teach us things that can aid in our understanding of galaxy formation; exoplanet transits give glimpses of what other planetary systems around other stars may be like.  Everything varies at some level or some stage of its lifetime, and these variations are key to understanding our ‘Restless Universe’.

Join us next time when we talk about the observations and discoveries being made by amateur and professional astronomers, and how you can contribute to science by observing variable stars.

365 Days of Astronomy
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The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.

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