Lepus

Lepus, the Hare.  Just South of Orion
with its Ears on the Top-Right

Lepus is a constellation that looks remarkably like its name.   It sits just south of Orion's feet with its ears sticking northward.   Alas most of the stars of the Hare are rather fainter than those in the neighbourhood of Orion to the north and Canis Major to the east.  Just to the south (below)  of Beta Lepus (or Nihal) lies the distant globular cluster M79.  It is nearly 60,000 light years from the galactic centre and over 40,000 light years from Earth.    What makes M79 especially interesting is that it was probably born not in the Milky Way but in the dwarf irregular galaxy in Canis Major.  This dwarf galaxy is the nearest to the Earth and is currently being shredded by the gravity of the Milky Way.

Best Views in the Solar System

Here are two great views in the solar system simulated using Stellarium.  Not exactly stars but pretty nice!

Mars from the Surface of Phobos

Saturn from the Surface of Mimas

Alkab or Iota Aurigae

Auriga labelled by Astrometry.Net!

I thought that we would spend some more time in Auriga since it is high up right now. It turns out the Gregory Chaucer (of Canterbury Tales fame) wrote a book about how to use an astrolabe.  An astrolabe is a navigational instrument that allows you to calculate the positions of the stars on the sky --- think about a medieval Celestia or Stellarium.   Alkab is a bright giant  of spectral type K, so it is the same colour as the nearby giant Arcturus but it is inherently much brighter at about 500 light years distant (Arcturus is only 30 ly away).

Alkaid's Neighbors

I mentioned Vancouver's zenith star, Alkaid, a few posts ago. In and of itself Alkaid is not particularly remarkable, but two of its neighbours in sky are. Within a couple of degrees of Alkaid are two of the most beautiful galaxies in the firmament, M51 (The Whirlpool Galaxy) and M101 (The Pinwheel Galaxy). Here are some photos from the Hubble Space Telescope (NASA/ESA).

M51 - The interacting Whirlpool Galaxy

M101 - The Pinwheel Galaxy

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

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.


View Larger Map

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.