Arcturus isn’t universally known as the “World’s Fair Star,” but it should be. Its light bridged two World’s Fairs, making an astronomical link between the one in 1893 and a second in 1933 – both held in Chicago. It’s intriguing that the general public was excited enough about science – in the middle of the Great Depression – to make such a link attractive to the Fair’s promoters. Light from Arcturus – believed at that time to be 40 light years away – was captured by the 40-inch refractor at Yerkes Observatory and its energy used to turn on the lights for the 1933 Fair.
This put the public spotlight not only on Arcturus, but it raised consciousness about the vast distance between us and that star, since the light being used had started its journey during the 1893 Fair and arrived just in time to start the next Fair. When you know light can circle the Earth more than seven times in a single second, you start to understand just what an incredible journey that was.
Of course Arcturus has many other distinctions. For one thing, it makes a perfect connection with the best known asterism in the sky, the Big Dipper. To find it, all you have to remember is “follow the arc to Arcturus.”
And in 2014 you get a bonus – keep following that arc and you’ll quickly come toa slightly brighter “star,” the planet Mars! More about that in our “events” post for April, but I did add Mars to this month’s “look east” chart. It forms a nice triangle with Arcturus and Spica, another bright star we’ll meet next month as it’s quite low this month.
Getting back to Arcturus – another way to remember where to find Arcturus is its name, derived from ancient Greek, which can be translated as “Bear Watcher.” That’s because Arcturus looks like it’s keeping an eye on the “Great Bear,” Ursa Major, as both circle the northern pole.
You can also think of the magnitude system by which we rate the brightness of stars as starting near Arcturus. At magnitude minus .04 it’s about as close to zero as you can get – the minus sign indicating it is a tad brighter than zero. Its absolute magnitude is also pretty close to zero since absolute magnitude is defined as how bright a star would be if it were about 33 light years from us, and by modern measurement Arcturus is now believed to be about 37.6 light years from us. That makes its absolute magnitude -.29.
Arcturus has the distinction of being the brightest star in the northern celestial hemisphere, but this is splitting hairs in several ways. It means it’s the brightest star north of the celestial equator. Sirius, now over in the southwest, is obviously brighter. But Sirius is south of the celestial equator. Both stars are located close enough to the celestial equator so they can be seen from most places on Earth.
It’s interesting to note, however, that Mars is outshining it this month- by quite a bit. In round numbers, Arcturus is zero, Mars is minus 1.2 and Sirius, setting in the est early on April evenings, is minus 1.5.
But Arcturus (-.04) also wins the “brightest star in the northern hemisphere” distinction by another hair. Remember that the lower the magnitude number, the brighter the star. Both Vega (.03) and Capella (.08) are north of the celestial equator, and the difference in brightness between Arcturus (-.04), Vega (.03), and Capella (.08) is roughly a tenth of a magnitude. For practical purposes, they all look the same. But in practical terms, making the comparison by naked eye is – well – very impractical. Capella is currently fairly high in the northwest. But next month, when Vega is high enough in the east to see well, Capella will be rather low in the northwest. At that time Arcturus should look brighter – but its actual brightness will be aided by the fact that it is high over head at that time, so you are seeing it while looking through a lot less air than you will be when looking at Vega or Capella. Besides, visually trying to compare stars that are this far apart in our sky is next to impossible since you have to look away from one to see the other. I simply think of all three as magnitude zero and leave the hair splitting to the scientists and their instruments.
Oops – we interrupt this program for a bulletin from 1907!
Yes, having just written how impractical the naked eye comparison is, I found this passage in “The Friendly Stars” by Martha Evans Martin, a book that was published more than a century ago:
Arcturus and Capella are so nearly equal in brightness that astronomers differ as to which outranks the other, even when they measure their light with a supposedly accurate instrument and a trained eye. To my own eye Arcturus outshines Capella, and on asking various inexperienced persons for off-hand opinions as to the relative brightness of the two stars, I have invariably had an answer in favor of Arcturus. The best authorities, however, make Capella a shade brighter.
Oh my! And now with 100 years of scientific progress, the verdict is that Martha Evans Martin and her causal observer friends were correct – and the “best authorities” wrong. Arcturus is the brightest. So much for my idea that you can’t tell the difference with the naked eye! Give it a try and see what you think. (You can find a chart for Capella and more details about that star in this post.) Since we’re ranking stars, however, Arcturus is actually fourth on the list of brightest stars – two others that are ahead of it, Canopus and Rigel Kentaurus, are not seen by observers in mid-northern latitudes. Sirius, of course, is.
While Arcturus radiates a lot of energy, much of it is not in the form of visible light. It has what’s known as a “peculiar spectrum” and radiates much of its energy in the infrared portion of the spectrum. This means that to our eyes it doesn’t look as bright as it really is.
One more deception of sorts: This brightness is not because Arcturus is so big – well , yes it is, but not big in terms of the amount of stuff in it, but big in terms of surface area. If you’re measuring the amount of stuff that makes up Arcturus – its mass – it is about the same size as our Sun. But Arcturus has a much greater surface area, so think of it as a hugely bloated version of our Sun. (Keep that in mind when you look at the comparison sketch above.) It is a much older star and is now going through its red giant phase, something our Sun will probably do several billion years from now, burning the Earth to a cinder in the process.
Hmmm . . . to get an idea of how much impact that large surface area has, if you put our Sun out near Arcturus it would be barely visible to the naked eye – and then under truly dark – not light polluted – skies.
Vital stats for Arcturus, also known as Alpha Bootes:
• Brilliance: Magnitude -.04, brightest star in the celestial northern hemisphere; shines with the luminosity of 215 Suns.
• Distance: 37 light years
• Spectral Type: K1 Giant
• Position: 14h:15m:38s, +19°:10′:5
Each month you’re encouraged to learn the new “guidepost” stars and asterisms rising in the east about an hour after sunset. One reason for doing this is so you can then see how they move in the following months. So if you have been following – even if this is just your second month – look for the previous guidepost stars and asterisms that you have learned and that are still with us in April. Here’s the list from east to west.
- Leo’s Rump (triangle)
- The Sickle
- the Beehive
- Orion’s Belt
- the Kite
- the Winter Hexagon
- the Pleiades