Wednesday 23rd July 2014
Here I give you the incredible facts behind some of the most interesting stars, and show you how pick them out for yourself in the sky tonight…
If you cast your eyes into the night sky tonight, all the stars are really just individual point sources of light of different subtle brightness and colours, and all inside our galaxy The Milky Way. Of course all together the stars in the sky make for a pretty fantastic skyscape, but there’s much more to those little points of light than meets the eye. Like in much of astronomy your imagination is valuable, you could never see up close the mind bending size of the red and blue supergiants, or get anywhere close to imagining just what the sunshine is like coming from a star 10,000 times more luminous than our Sun. The stars come in all different sizes, colours, brightness, temperatures, including some that look like one star but is in fact a system of two, or even four stars orbiting each other.
Times are GMT, one hour is added for BST between March 25th and October 28th
The star Arturus is a red giant, 25 times the diameter of the Sun, and travelling at some pace through the Galaxy. If you look quite high to the west at nightfall you’ll see the unmistakable shape of the Plough, or The Big Dipper, whatever you prefer. If you imagine a line that follows the curve of the handle, you’ll come to a bright orange star 30 degrees from the Plough handle end star.
This is Arcturus, the brightest star in its constellation Bootes, and one of the brightest in the sky. Arcturus was originally a star not unlike the Sun but is further along in its lifetime, as it has now evolved into a red giant. Arcturus has swelled up to a bloated size as it comes to the end of its stellar fuel, and will eventually cast off its outer layers to become a planetary nebula, just as our Sun will do in a few billion years time. Arcturus is emitting about 110 times as much visible light as the Sun, but in infra-red it is even more powerful at around 200 times stronger. This stellar specimen 36.7 light years away is a few thousand degrees cooler than the Sun, and likely much older.
This star is moving at a rapid speed through the galaxy, and in a weird direction. It is travelling at a speed of 122 kilometres per second not in the general direction of all the rest of the stars, gas, and dust, but doing its own thing going perpendicular through the plane of the Milky Way. It has company too, Arcturus is one of about 50 other stars moving in the same way, in a stellar stream called the Arcturus Stream. Streams of stars like this are caused by gravitational tidal forces, stretching and pulling collections of stars from their original formation into long tendrils.
Arcturus is at magnitude -0.04
Where and when…
To find Arcturus follow an imaginary curving line from the handle of the Plough, to a distance of 30 degrees from the handle end star (Alkaid). You’ll come to a bright orange star, this is Arcturus in Bootes
Tonight you’ll see this star fairly high in the south west at nightfall, it then sinks low to the west before sunrise.
Coordinates for Arcturus RA 14h, 16m, 14s /DEC 19, 07′, 24″
If you’ve got an unobstructed northern horizon, after dark look very low for a bright star and you might see the bright star Capella in Auriga. Capella is actually the sixth brightest star in the entire sky. Auriga is between constellations Orion and Perseus along the plane of the Milky Way. Look at Capella and yes it looks like your pretty normal, brighter than average star twinkling away. But all is not what it seems, Capella isn’t one, Capella is four, it is a star system of two binary pairs.
Capella’s clutch of four Suns come in two different flavours, a main pair of large G type stars measuring 12 solar radii…and a pair of faint, cooler red dwarfs. The main pair, and the red dwarfs make two binary pairs orbiting around each other. The main pair are in the same class of stars as our own Sun (G type), and orbit close to one another (100 million kilometres), whereas the red dwarfs lie some distance away. Quite far away in fact, the red dwarfs lie at around 10,000 astronomical units from the main stars, that’s 10,000 times the distance of the Earth from the Sun. Yes, space is big!
The main large pair each weigh in at roughly 2.7 times the mass of the Sun, and kick out 79 times more sunshine. They orbit each other once every 104 days, with about 100 million kilometres between them.
The second pair, being red dwarfs will have a mass of less than half that of the Sun’s. These smaller, cooler stars are the longest lived stars in the universe, so long lived in fact that the cosmos hasn’t even been in existance long enough for any to come to the end of their lives. So no red dwarf star has ever yet been observed to die, as they are slow burning, they really do take their sweet time. Red dwarfs can also emit as little as 1/10,000th the light of the Sun, so any planets in orbit around them need to huddle in close to warm any potential life there.
So there it is, the ‘star’ Capella in Auriga, 42.2 light years from Earth, and a little bit more to it than you might have first thought.
Capella is at magnitude 0.05
Where and when…
Tonight look north and you might see see Capella very low to the northern horizon after night fall.
Coordinates for Capella RA 5h, 17m, 36s / DEC 46, 0′, 37″
If you look quite high to the east at night fall you’ll see the fifth brightest star in the sky Vega rising in constellation Lyra . It lies adjacent to the distinctive shape of the swan of constellation Cygnus. Vega is one of the three bright stars that make up the summer triangle, together with Deneb in Cygnus, and Altair in Aquila. The summer triangle is an imaginary shape drawn between these stars against the celestial sphere, and is high up in the sky for northern latitudes during summer.
Compared to the Sun Vega is much, much younger at only one tenth the age of our own star. That makes Vega under 500 million years old, forming out of its gas cloud into a brand new star, just as complex life was starting to really get its act together on Earth. But Vega is not only younger than the Sun but has also 2.1 times more mass, meaning Vega will also die at a younger age. This is the fate of the larger higher mass stars, they have a brief and often violent life, going through their fuel at a quicker rate. In fact Vega is evolving so fast that it could already be nearing middle age, these heavier stars live fast and die young. Vega at 25.3 light years away is also bigger than our Sun, at over 2.7 times its radius.
All that is very interesting but Vega is doing something else, that you might not expect when you see it sitting there apparently unchanging and motionless in the sky. This star is spinning so fast it’s just under the point of breaking apart, it’s at 93% of that point. Just like how our very own Jupiter bulges out at the equator and squashes at the poles from its fast rotation, so it is the same for Vega. The equator of Vega is racing around at a speed of 274 kilometres per second, and the whole star rotates once on its axis every 12.5 hours. As you look up at the star in the night sky, you’re actually looking down onto its pole, so Vega’s axis of rotation is is pretty much pointing directly at you. This means that from your perspective on Earth if you could actually see it, you’d see Vega’s equatorial bulge. It’s large at a massive 23% increased diameter across its equator, than its poles.
Vega is at magnitude 0.00
Where and when…
Tonight Vega is in the constellation of Lyra climbing in the east after dark. It is still high in the south west sky by daybreak.
Coordinates for Vega RA 18h, 37m, 21s / DEC 38, 47′, 43″
Mizar (and Alcor)
Look up to Ursa Major tonight in the north west after dark, and you’ll find the famous and easily recognisable asterism of the Plough, also known as the Big Dipper. The Plough is not the whole Ursa Major constellation (The Great Bear), but forms only a part of it. Also the seven stars of the Plough are not physically related to each other, but appear to be because of their similar brightness forming the well known shape.
Ok so as you’re looking up at the Plough, see the second star in from the end of the handle? That’s Mizar, at just over 78 light years away. But keep looking and you should see another less bright star pop into view next to it, that’s Alcor at just over 81 light years away…3 light years behind Mizar. So the star Mizar in the Plough handle is actually a double with Alcor. This easy naked eye double was thought to have once been used as a test for good eye sight. If you can see both stars then you’ve got pretty good vision, although spectacular eye sight is not required as most people can pick out both Mizar and Alcor quite easily.
But there’s more to these two stars, more stars in fact. If you look at Mizar through a telescope you’ll see that it itself is actually a binary double system. Bright Mizar has a nearby fainter companion star orbiting around it, Mizar A being the main star, and Mizar B its companion. Stay with me here, as it goes even deeper. Both Mizar A and also its fainter orbiting companion Mizar B even have their own companion stars orbiting around them as well. But these extra stars are so close that they appear as just the one star known as a spectroscopic double. But what about the other star in the main pair, Alcor? Well it was also found that Alcor has a companion, a star also orbiting so close that they just appear as the one, another spectroscopic double. So Mizar, the second star in the Plough handle is in fact really a double, double, double, six suns all gravitationally bound together in a sextuple system. The Mizar star sytem is one of only two six star systems in the whole sky, the other is Castor in the constellation Gemini.
Mizar is at magnitude 2.2
Where and when…
Tonight as darkness falls you’ll see the Plough moderately high in the north west after dark. It then sinks lower towards daybreak.
Coordinates for Mizar RA 13h, 24m, 25s / DEC 54, 51′, 42″
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