Algol

Algol in Perseus

Algol is the classic example of an eclipsing binary.  It is also known as the Demon Star.  Every two days, twenty hours and 49 minutes, its brightness dips by a factor of three for about ten hours and the secondary star eclipses its brighter companion.

The more massive star of the pair is still on the main sequence, but the less massive one has already evolved to become a subgiant.   This presents a paradox because usually more massive stars evolve faster.   The solution is that Algol B has lost mass to Algol A.   The current secondary star was at one time the more massive and as it evolved off the main sequence and swelled it lost mass to its companion and become the secondary star.

When a star loses mass to its partner, not only does the mass transfer change the stars themselves, it also changes their orbit.    When the more massive star transfers mass to the less massive star, their masses come closer together, and also their orbit shrinks bringing the stars themselves closer to together.   As you might imagine, this will encourage further mass transfer.  

One would think that this process would end when the two stars had equal masses.   Thereafter, further mass transfer would cause the orbit to grow.  This is correct; however, the donor star could swell further (and even swell as it loses mass) so the mass may continue to flow from the less massive star to the more massive star, even though the orbit starts to grow again.  This is probably what happened with Algol A and B.   By the way there is also an Algol C that orbits around the other two stars with a period of nearly two years --- it is quite far away compared to the others.  Its orbit would remain more or less the same in spite of the shenanigans of the other two stars.

Perhaps I will catch a fainter Algol in a subsequent photo --- I have a one in seven chance!

Almaaz or Epsilon Aurigae

Almaaz and Auriga

Right next to Capella in Auriga lies Almaaz (or Epsilon Aurigae).  Almaaz is a strange beastie.  It is an eclipsing binary.  Every 27 years the brightness of the binary drops by factor of two for nearly two years.   Its variability was discovered over 150 years ago but only recently has some consensus grown over the nature of the companion that blocks the light.

In 2009 observations with the Spitzer infrared telescope have found evidence that the primary is post-asymptotic giant star (like Mira but further along -- its pulsation period is about 66 days, much shorter than Mira) and that the secondary is a  B-star with a disk of material around it.   The system is actually in eclipse right now from 2009 to 2011.

The American Association of Variable Stars (AAVSO) have made it a special target during the current eclipse and you can check out a light curve up to today at this link.  Really Cool!

Alkaid

The zenith star for Vancouver lies at the end of the handle of the Big Dipper, Alkaid (or Alcaid).    It is actually brighter than its handle-lighting compatriot at the end of the Little Dipper, Polaris.   Mirphak (α Per is a bit too far north) and Capella (α Aur is a bit too far south) come close, but Alkaid works best. Alkaid, also known as Benetnash, is just a smidgen over 100 light years from Earth. It is a star about six times the mass of the Sun, not massive enough to produce a supernova but still very short lived. It is still along the main sequence consuming hydrogen in its core and shining blue-white. It is otherwise relatively unremarkable.

Capella

Capella and the Helmet of Auriga

The binary-binary star Capella is the bright dot at the top of the image.  If you zoom it, you will see it is quite orange-yellow.  The brighter pair of stars are two yellow giant stars (a bit less orange than the nearby giant Arcturus).   The more massive of the two is slightly more evolved on the cusp of being called an orange giant.  It has probably already begun combining helium to make carbon in its core. The second pair of stars are two red dwarfs about 10,000 AU from the first pair.   The giants are a strong x-ray source about four orders of magnitude brighter than our Sun.

With these two giant stars orbiting so close to each other at about 100 million kilometers, they is a lot of action to look forward to, if we could only wait a few dozen million years as the envelopes of both stars swell to surround the entire binary.   In the meantime, we can watch for the interactions of the strong winds from the two stars that possibly play a role in the x-ray emission from this first stellar x-ray source to be discovered.

Mira

Mira and Much of Cetus

Mira means "wonderful."   Mira is one of the few variable stars that is visible at its peak and too faint to be seen at its minimum.   It was the first, non-nova variable star to be discovered.  It so happens that Mira is near its maximum now.  It is indicated at the end of the arrow.  If you zoom in, you can make out its reddish colour.   Mira is now around 3rd magnitude about as bright as the rest of the stars in the constellation Cetus.   Its maximum lasts a few weeks and then Mira will drop below tenth magnitude, fainter than the faintest smudge in the image.

It reaches its highest point in the sky around 2am, so it is understandable if you miss it this time around.  It will be bright again in 11 months and will reach its high point around 4am -- maybe a bit more convenient.

Aldebaran

Aldebaran, the Hyades and the Pleiades 

Aldebaran is the brightest star in the constellation Taurus at the head of the bull (in the lower left corner of the image).  Although in the image, Aldebaran appears white, it is an orange giant like Arcturus.  It has exhausted the hydrogen in its core but has not yet begun fusing helium.  Aldebaran appears to be a member of the Hyades cluster of young stars at the head of the bull, but it is actually much closer at 20 pc.   Because Aldebaran lies near the ecliptic, it is often occulted by the moon; this has given us many measurements of its angular size of about 20 thousandths of an arcsecond so a diameter of about 40 times that of the Sun.   This measurement provides a calibration for stellar models and interferometric studies of other objects.

A cool question to ask is whether we could detect planets orbiting these giant stars.   Assef, Gaudi and Stanek (http://adsabs.harvard.edu/abs/2009ApJ...701.1616A) argue that although the duration of the transits is long (about 50 hours or longer) and the amplitude is small (less than one part per thousand), the systematics of the photometry can be kept under control that even one-meter telescopes could discover such transiting systems from the ground.   From satellites such as Kepler, COROT and MOST the prospects are even better.

Rigel

Let's stick with Orion a bit longer.   The star Rigel is usually the brightest in Orion (even though it is sometimes called β Ori). In fact Rigel is further from us that Betelgeuse, and it is indeed the brightest star within 1,000 light years of Earth. Rigel like Betelgeuse is a supergiant star, but it is blue rather than red. Rigel intersects my work because it too has been detected as a radio source, so it gives us an idea of which blue stars will be visible with the Square Kilometer Array. It turns out for various reasons that although Rigel and Betelgeuse are inherently much brighter, the stars like Arcturus are much better prospects for the Square Kilometer Array. Giant stars like Arcturus are much more common than supergiants, so the SKA should see lots of them even in the bulge of our Galaxy. The Wotjobaluk koori of southeastern Australia held that Rigel was the mother-in-law of Altair which explained why there are rarely seen in the sky together.

Betelgeuse

Orion with Betelgeuse at top left and Rigel at lower right

Betelgeuse is once of everyone's favourite stars, at least to say.  This red supergiant forms the left shoulder (from our point of view) of the constellation Orion, the hunter.  Betelgeuse was discovered by Herschel to vary in brightness; sometimes it is actually brighter than Rigel, the blue supergiant at the lower right of the constellation.  Betelgeuse along with the three blue stars of Orion's belt all probably escaped from the Orion OB association that lies in the molecular clouds along Orion's sword.   Unlike in most constellations many of the bright stars of Orion are actually associated with each other.

Betelgeuse is a relatively close yet physically large star extended to a radius of a few AU (Mars's orbit), so the stellar disk of Betelgeuse has actually been resolved at visible wavelengths (like Altair) , and it is also a radio source (like Arcturus), that I used as an example to understand the radio emission from stars in the Galactic bulge and what might be seen with the Square Kilometer Array.

The photosphere of Betelgeuse appears to shift and pulsate, so it has been difficult to measure its distance using parallax.   The theories of such giant stars is that the extended photosphere of such giants is convective with only a few cells (the Sun has thousands).   The heat transport through the extended, tenuous photosphere is extremely inefficient, and the density is so low that it makes sense to call its atmosphere a hot vacuum.   It has recently been discovered that this convective atmosphere efficiently generates a magnetic field (http://adsabs.harvard.edu/abs/2010A%26A...516L...2A).

The Planets Move

Jupiter and Uranus at 9 pm PDT on 2 October 2010

Jupiter and Uranus at 10 pm PDT on 4 October 2010

I shared a nice photo of Jupiter (and Uranus) from a few nights ago. I took another photo of the same area of sky last night. I have rotated both so that west is to the right and both have the same scale. It is quite obvious that Jupiter over the course of just two nights has moved closer to the 5.45-mag star on the right (20 Pisces). The length of the stellar stripes is very closer to 450 arcseconds (because Jupiter is along the celestial equator now), so you can figure out how far is has moved. It also has moved north a bit following the ecliptic. Less obvious is that the stripe at the top of the image has also moved west a bit too, so my claim that it is Uranus seems to hold water! Unfortunately, it was a bit cloudy and I shook the tripod at the beginning of the exposure, so the second photo isn't as nice. I try again soon.

Of course, none of this is surprising except for the fact that one could do so well with such simple equipment.

Altair

When I went observing in Armenia in 1992, Armenia and Azerbaijan were fighting.    Just south of the observatory was the border between Armenia and the Azeri exclave of Nakhchivan.  Just to the east of the observatory was the exclave of the exclave, the village of Karki that had been captured by Armenia just a couple of months earlier in May 1992.   Anyhow, one night mortars knocked out the power to the telescopes, cutting off the night's observing.   I had been working through the day, so I was already asleep and I slept through the excitement.

The previous star I mentioned was Vega.  Just across the Milky Way lies α Aquilae or Altair.   Altair like Vega is a bright blue star.  In fact it is almost a twin of Vega down to its rapid spin.   The biggest difference is that we are looking at the equator of Altair more or less (while we see Vega's pole), so where Vega was brighter from our point of view than average, Altair is fainter.  Altair is one of the few stars for which we have made a direct image using optical interferometers.  It is also by far the smallest in angular size.  Not surprisingly Altair is quite squished, as Vega is, but in the case of Altair our viewpoint is favourable actually to see the distortion.  For Vega we can only infer the shape.

Here is a photo of the Eagle (Aquila) from tonight with Altair as the eye of the bird near the top of the image (just to the left of the word "Altair").  It is a 30-second exposure with an five-year-old digital camera.

Aquila (the Eagle) with Altair labeled

Here is a photo of Jupiter and the Galilean moons (from left to right: Io, Europa, Ganymede and Callisto) taken a couple of minutes earlier.  The planet Uranus is the blue stripe in top-left corner.

Jupiter and the Galilean Satellites with Uranus at Top Left