Vega

Eighteen years and a month ago I spent a few weeks on a mountain in Armenia near the points where Iran, Armenia and Azerbaijan meet.   I was there with a group of astronomers from the Pulkovo observatory in St. Petersburg to observe spectrophotometric standard stars.  I would swing the telescope from α Aquilae to α Lyrae who straddle either side of Cygnus, the swan.  We looked at other stars that I can no longer remember.   The common name for α Lyrae is, of course, Vega, and Vega is the ultimate standard star that forms the basis for astronomical measurements at least in the optical and nearby.   The apparent brightness of other stars is compared to Vega, so Vega is an especially well-studied star.


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Vega is one of the brighest stars in the northern sky and from Vancouver where I'm sitting, it is visible most nights -- a blue-white beacon in sky.   Vega is about twice the mass of the Sun, so it should be about 20 times brighter when in fact it appears to be nearly sixty times brighter than the Sun, much brighter than other stars of its mass; Sirius for example is 25 times brighter than the Sun.   This presented quite a puzzle until it was discovered that Vega is rotating once every 12 hours or so.  If you looked at the star from the side, it is squished like a melon --- if it rotated just a bit faster, material would fly off of its equator.   We are looking down on the pole of Vega so it is difficult to measure the rotation velocity, but because the pole is closer to the centre of the star, the gravity at the pole is stronger.  It turns out for a particular star, regions where the gravity is stronger put out more light, so from our point of view Vega is about 50% brighter than averaged over the surface.   The rapid rotation makes Vega somewhat brighter than a similar slowly rotating star from every direction.

Gliese 581

Gliese 581 is a red dwarf star about twenty light years from Earth in the constellation Libra.  It is about one hundred time fainter than the faintest stars that we can see with our eyes and about one third the mass of the sun.  So what makes it special?   It turns out that Gliese 581 has six (and counting) planets in orbit around it.   Last to be discovered, Gliese 581g, is at just the right distance from the star so that its surface temperature would allow liquid water, presumably one of the prerequisites for life.  It turns out that this planet is quite interesting.  Its rotation is tidally locked with its orbit, so as one side of the Moon always faces the Earth, one side of the planet always faces its star.  The most comfortable place to live would been along the terminator between night and day.

The planet orbits the star every 37 days at a distance seven times closer than the Earth is to the Sun.  The mass of the planet is three to four times that of Earth, so astronomers guess that it may have an atmosphere with temperatures ranging from -32C on the night side to 71C on the day side.

http://arxiv.org/abs/1009.5733 The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581

http://www.santacruzsentinel.com/localnews/ci_16212943 UCSC astronomer, others discover first habitable planet outside our system

Procyon

Canis Minor with Procyon at the Bottom

Procyon (at the bottom of the poor photo or at the top of the Sirius photo) is the nearest neighbor to the Sirius system, and it is also a binary star with a white dwarf.  Procyon's white dwarf is somewhat less massive at 0.6 solar masses and quite a bit older too.    Procyon A itself is somewhat more massive than our sun and has evolved a bit off of the main sequence.   Procyon means "Before the dog" because it rose before its neighbour Sirius (a.k.a. the dog star) about 2000-3000 years ago along the Mediterranean.   Nowadays Sirius actually rise first because the Earth's rotation axis has shifted a bit with respect to the stars.   Whence all of the dog talk, Sirius is the brightest star in the constellation Big Dog, and Procyon is the brightest star of the Little Dog.   Procyon is also the name of the genus of animals that contain the raccoon who I guess comes before the dog as well.  Procyon and Sirius were terribly important to the Egyptians as Sirius rises just before the Sun when the Nile is about to flood.  Because at the time Procyon led Sirius the rises of Procyon just before the Sun gave the Egyptians some advanced warning.

The MOST satellite watched Procyon for a month in 2004 and contrary to expectations did not find oscillations it in flux.   Many stars including Procyon are thought to have convection regions within (thinking the roiling of the boiling pot of water).  The roiling motion causes the brightness of the star to vary slightly.  Astronomers thought that the variation would be strong enough to be see with MOST but it wasn't.  This was somewhat surprising and is still not well understood.

Sirius

Canis Major and Canis Minor

Sirius (at the lower right of the photo) of course is the brightest star in the sky after our Sun, and being an astronomer it provides me with a lot of interaction from the general public.   Every spring it seems I get several calls from people saying that they saw some sort of UFO in the southwestern sky in the evening.  It blinks and flashes different colours at them and appears to follow them.  With a little investigation, the object is identified as Sirius.   When it is higher in the sky in the early spring it is rarely unidentified. As I mentioned in an earlier post, Sirius was used by the Polynesians to navigate.  In fact it is the zenith star of Tahiti.   That means that if Sirius passes directly over your head, then you at at the latitude of Tahiti.  Quite useful if you are lost on the Pacific Ocean.

Sirius is actually two blue-white stars in orbit around each other.  The brighter is a main sequence star about twice as massive as the sun.  The fainter is a white dwarf star about as massive as the Sun but with the radius of the Earth.  Sirius B presented a great problem for physics that was only solved by quantum mechanics, nearly a century after its discovery.   Sirius A and B are two of the eight closest stars to the Sun, so white dwarfs clearly a common in the sky which only made the problem more frustrating. Although Sirius B is much fainter in the optical, it dramatically outshines Sirius A in the x-rays.  And there is possibly a habitable zone for planets that orbit around both stars but no potential Tatooines have been discovered.

Arcturus

Arcturus Setting just above the Trees

As I write this in late September, the reddish star, Arcturus, sets soon after the Sun, so it isn't nearly a dazzling at mid-summer when it reaches its peak in the early evening.  It lies just above the trees in the photo with the rest Bootes extending upward. But that is not why I'm writing about it.  At eleven parsecs Arcturus is the nearest giant star to Earth, and because its mass is similar to the Sun, it gives a picture of what the Sun might look like in five billion years.   Because it is so close and a giant star, it is one of the few normal stars from which we can detect radio emission.   This emission is analogous to the radio waves from the quiet sun, but because Arcturus is so much larger than the Sun (about twenty solar radii) its radio emission is inherently much brighter.

This got me thinking about all of the stars like Arcturus and further evolved in the bulge of our Galaxy. These are the sources for surveys of gravitational microlensing such as OGLE.  How would they appear if we observed a microlensing event in the radio?  It turns out that about eight percent of the OGLE-2 sources could be detected by a planned radio telescope called the Square Kilometer Array, and if the lens had a small enough mass, the radio flux would oscillate as the lens passed in front of the source, telling us the mass of the lens.   Pretty neat!  If you want to learn more, read http://arxiv.org/abs/1002.3007.

Of course, being the brightest star in the northern half of the sky, there are lots of stories about Arcturus from how its light opened the world's fair in Chicago to various myths.   The one that I found most interesting is how the declination of Arcturus is the latitude of the island of Hawaii, so when you are at the latitude of Hawaii, Arcturus passes directly overhead.   The Polynesians would sail northward from Tahiti and the Marquesas islands until Arcturus was directly overhead at its highest point and then sail west until they landed at Hawaii.  They called it Hōkūleʻa, the "Star of Joy", so when you look at Arcturus, you can imagine it at the zenith above a beach on Hawaii, and you too will feel the joy.  By the way, the Polynesians used Sirius for the return trip.

Each Star is a World

When I was growing up, I became interested in astronomy by looking at the night sky.  Fortunately, where I lived I could see many stars, and I learned their names and a bit about each of them.   I recently found a book,  The Hundred Greatest Stars, that gives a lot of information about 100 stars, but many of these stars are too faint to be seen.  I wanted to collect information about the stars that anyone familiar with the night sky knows.   Something like The Friendly Stars but of course updated with the modern information.   We know so much more about the bright stars of the night sky than Ms. Martin could write about at the turn of last century, so here goes.