Date: June 7, 2009

Title: The Brightest Stars In The Sky

Podcaster: Mike Simonsen of AAVSO

Organization: AAVSO www.aavso.org

Description: In today’s episode we discuss the brightest stars in the sky, and the little known fact that about one in three of them is actually a variable star. Some of them vary so subtly you can’t see it with the naked eye, yet at least one of them may blow up one day and be visible in broad daylight!

Bio: The AAVSO is an international non-profit organization whose mission is: to observe and analyze variable stars; to collect and archive observations for worldwide access; to forge strong collaborations between amateur and professional astronomers; and to promote scientific research and education using variable star data.

Today’s sponsor: This episode of ‘365 Days of Astronomy’ is sponsored by Doc Kinne in recognition of his astronomy professors and advisors at Swinburne University of Technology, Victoria, AUS. 365 Days of Astronomy is not enough time to celebrate what you folks do.

Transcript:

Hi, and Welcome to Restless Universe, the podcast of the American Association of Variable Star Observers. You can find us on the web at www.aavso.org. I’m Mike Simonsen.

“Star light, star bright, first star I see tonight…”

On many nights, the first star you’ll see, as the sky grows dark is actually a planet, usually Venus, Jupiter or Saturn. If you want your wish to come true, you should keep track of these bright imposters, so you can actually make your wish upon a star. When you do settle on a real star to make a wish on, it is probably going to be one of the top ten or twenty brightest stars in the sky. And whether or not you know it, chances are pretty good that bright wishing star is also a variable star.

Eight of the 25 brightest stars are variable stars. Five out of the top 15 brightest stars are variable, so the first star you make a wish on tonight has a one in three chance of being a variable star. These bright stars are the subjects of today’s podcast.

The brightest star in the night sky is Sirius, also known as the ‘Dog Star.’ Although Sirius is not a variable star today, it has been in the past, and will be again in the future.

Sirius is actually a binary star. Sirius A, the main component is a hot bluish-white star, roughly twice as massive as the Sun. Sirius B is a white dwarf, almost as massive as the Sun, but only as large as the Earth. White dwarfs are the remains of highly evolved stars that have already gone through the red giant phase and lost their outer atmosphere to space, or in this case, a close companion. Sirius B started out as a star five times the mass of the Sun, but it’s been a white dwarf for 120 million years now. Eventually, Sirius A will also leave the main sequence and become a red giant, then ultimately, a white dwarf.

Arcturus, the fourth brightest star in the sky, is an orange colored giant star that is on the borderline between true variability and being constant. The redder stars are, placing them close to or within the spectral class M, the more they vary in brightness due to pulsation. Arcturus is spectral class K, 3.5 times as massive as the Sun and 25 times the radius of our modest star. According to photometry performed by the Hipparcos mission it varies by about 0.04 magnitudes in 8 days.

Betelgeuse, the ninth brightest star in the sky, marks the upper left shoulder of Orion and looks reddish to the naked eye. That’s because Betelgeuse is a red supergiant star. In fact it’s one of the largest stars known. Placed at the center of our solar system, Betelgeuse’s surface would be somewhere between the orbits of Mars and Jupiter. Betelgeuse is a semi-regular variable star, varying in brightness between 0.3 and 1.2. Betelgeuse is a relatively young star, but has evolved quickly due to its 20 solar masses. Massive stars tend to live fast and die young. In the future, Betelgeuse may end up as a supernova.

If it does, it will be the brightest supernova ever seen by man, easily outshining the Moon. It will be visible in the daytime for several months. For now, we’ll just keep an eye on its subtle, unpredictable variations, waiting for a hint of what’s to come.

Beta Centauri, also called Hadar, is the second brightest star in Centaurus. Like Sirius, Hadar has some surprises in store the closer you look. Hadar is actually a triple star system. The primary and secondary are separated by a mere 1.3 seconds of arc. The primary is also a spectroscopic binary. One or both of the components of the primary is a variable star, a Beta Cephei type variable. These are pulsating variables with subtle light variations typically 0.01 to 0.3 magnitudes with periods of 2 to 12 hours. They are 10 to 15 solar mass main sequence stars. The point of maximum brightness roughly corresponds to the maximum contraction of the star as it pulsates.

The thirteenth brightest star in the sky, Aldebaran is the ‘Eye of the Bull’, and the brightest star in the constellation of Taurus. Aldebaran is a K5 star, which means it is a red supergiant star that has evolved off the main sequence after exhausting the hydrogen fuel in its core. It has a smaller companion, a dim red dwarf orbiting at several hundred astronomical units. Now primarily fusing helium, the primary star has expanded to a diameter of 44 times the diameter of the Sun. It is an irregular variable with a range of about 0.2 magnitudes.

The next brightest star in the sky, Antares, is an M class red supergiant, 700 times the radius of the Sun. It is the brightest star in the constellation Scorpio. For its size, this star weighs in at a mere 15-18 times solar, so it has a very low average density.

Antares is also a double star. Its companion is a hot, blue, magnitude 5.5 star. It’s difficult to observe in a small telescope and if you do manage a glimpse of it on a good night it may actually look green due to the contrast with the bright red primary. Antares is a variable star, whose apparent magnitude varies irregularly from 0.9 to 1.8.

Spica, also known as Alpha Virginis, is the brightest star in the constellation Virgo. To the unaided eye Spica shines constantly at magnitude 0.98, making it the 15th brightest star in the sky. But like so many other stars you wouldn’t suspect are actually variable, Spica is much more than meets the eye.

Spica is an interesting case, because not only does it pulsate, causing it to fluctuate in brightness, it is part of an exotic pair of stars doing a cosmic dance in very close proximity to each other.

Spica is an ellipsoidal variable. Ellipsoidal variables are binary systems where the two components are close enough to distort their shapes into elongated, egg-shaped stars. As they rotate around each other they show us varying amounts of combined surface area. When we see them both from the side we see the maximum amount of surface area. Since brightness is directly related to the amount of surface area throwing light our way, we see the pair at its brightest when we see them both from the side. When they rotate around to the point we are looking at them from the end of one or the other star, we see the least amount of surface area shining at us, so the star appears slightly dimmer.

The time it takes to go from bright to faint and then back to maximum light tells us the period of rotation. For Spica, that’s about four days. Spica varies from 0.92 – 1.04 magnitudes, too small a change to record visually. These variations are only detectable using photoelectric photometry or a CCD.

That might seem like a small, insignificant thing to note or study, but consider these facts. The primary component of the pair is 11 times more massive and 8 times the radius of our Sun. Its smaller partner is 7 times as massive as our sun and 4 times as big. These two massive stars spin around a common center of gravity so quickly that it only takes 96 hours to complete a cycle. Their combined light output is on the order of 15,000 times the luminosity of our Sun. It’s a good thing Spica is a comfortable 260 light years away, or there would be no night time on earth.

Not only does Spica have all this craziness going on, but due to instabilities in its interior, the primary has reached a point in its evolution where it is now pulsating. Its atmosphere is actually expanding and contracting periodically. The primary is a Beta Cepheid variable, like Hadar.

In future episodes, we’ll discuss pulsating stars with much larger amplitudes, like Mira and chi Cygni, both of which are visible to the naked eye when at their brightest.

Next month, we’ll talk about another interesting naked eye variable star, epsilon Aurigae. This intriguing star undergoes an eclipse once every 27 years, and this August is when the next eclipse is predicted to begin. Stay tuned to find out how you can help solve one of the great mysteries of our Restless Universe.

Until then, thank you for tuning in. Goodbye.

End of podcast:

365 Days of Astronomy
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