Where are all the stars in summer?
You can’t miss it on a balmy summer evening… glance upward, shortly after sunset, and you’ll see a distinctive triangle shape if you connect three of the brightest stars overhead.
This triangle shape, also known as The Summer Triangle, is what we call an asterism. In other words, it is not an official constellation, it is a made-up shape that we are ascribing to the heavens, to create a common frame of reference. The three stars, Deneb, Vega, and Altair, all belong to separate constellations… Cygnus, Lyra, and Aquila, respectively.
The first star, Deneb, is unique. It has a certain property that is different from almost every other star you can see (with your naked eye) in the sky. This physical property is apparent brightness, how bright a star appears to an observer here on Earth. Stars are governed by the “one over d squared” property… in other words, light falls off as one goes farther out by a factor of one over the distance from the star squared. Translation: the intensity of starlight falls off very rapidly the farther away you go. If you journey twice as far away, the light you now receive is only ¼ what you received at your original point (22 = 4). If you journey three times as far away, the light you now receive is only 1/9th the light you received at your original point (32 = 9).
As a result, all the stars you can see with your naked eye are at a maximum distance of around 700 – 800 light years. To see more distant stars requires either a telescope or a very bright star.
And Deneb is just such a star… quite a lot further than its summer triangle mates… at roughly 2,600 light years away! (Schiller, 2008) That is more than three times the max limit of naked eye stargazing. The distance to Deneb is under some dispute and astronomers are still trying to nail down an exact estimate. Other estimates are closer to 1,500 light years, still twice the maximum naked eye limit. It is amazing that we are even able to see this star, as far away as it is. Deneb achieves this seemingly insurmountable goal (of being visible from such a vast distance) by being one of the most intrinsically bright stars in our entire galaxy.
If Deneb were suddenly switched with the Sun (through some god-level magic) the Earth could be as far away as Pluto and still receive five times more light and heat than we do at our current distance (1 AU or approximately 93 million miles). Deneb is an extremely hot star… Type A2 on the spectral scale that astronomers use, at least 200 times larger (radius) than our Sun. Its surface temperature is around 8,500 Kelvin (14,840° F. For comparison, our Sun’s surface temperature is 5,800 Kelvin or 10,000 degrees F). Since its distance is still somewhat uncertain, we cannot be sure of its exact luminosity, but it could be as much as 200,000 times brighter than our Sun.
Deneb is my absolute favorite star. I love it because it is so brilliantly luminous and fascinating (I haven’t told you everything about it yet. I could go on, but then I wouldn’t be able to finish the tour!). I am already sad, because Deneb (due to its high mass and therefore limited lifespan) will almost certainly explode in a powerful supernova sometime during the next two to three million years or so. However, the material that is spread out from the blast will form new generations of stars… and that makes me very happy.
The next star on our tour, Vega, is not quite as bright (only 50 times the brightness of the Sun), and much closer (25 light years) at spectral type A0. As viewed from Earth, Vega is the fifth brightest star in the night sky (appearing to be brighter than Deneb since it is so much closer). Vega is a beautiful blue-white star and is also a fast-rotator (236 km/s velocity at equator (Yoon et al., 2010)), causing it to bulge out slightly at its equator.
Around 12,000 BC, Vega was the northern pole star (occupying the space (from our point of view) currently filled by Polaris), and will be so again in about 12,000 years. This is because the Earth’s axis wobbles a bit, as it revolves and orbits around the Sun.
Altair, another A type star (Type A7) is the closest of the three Summer Triangle stars, at only 16 light years distant. It is the faintest of the three (10.6 times the Sun’s luminosity (Peterson et al., 2006)) and only about 1.8 times the Sun’s mass (Monnier et al., 2007). Altair is another fast-rotator (286 km/s… even faster than Vega, also causing it to bulge out at the equator (Monnier et al., 2007)).
Tired of blue or white stars? Want to glimpse a true orangish-red giant star? Look no further than Arcturus. Find the handle of the big dipper, and then follow its arc (“Arc to Arcturus”, as the saying goes) until you reach it. Arcturus is roughly 37 light years away, and only 1.1 times the mass of the Sun (Ramirez, Prieto, 2011). It spreads its mass out over a much larger surface area, however, and is roughly 25 times the size (radius) of the Sun! (Ramirez, Prieto, 2011) Since it spreads its mass out over such a large area, the result is that the star has a much cooler surface temperature than our Sun… Spectral type K1 at 4,286 K (Ramirez, Prieto, 2011) or about 1,600 K cooler… and this is what gives it its splendid orangish-red color.
Now that we’ve seen Arcturus, we can “Spike to Spica”, a blue-white star (Type B1) and one of the 20 brightest stars in the night sky, by following a straight-line path out from Arcturus. Spica is pretty far, at 250 light years distance. Its distance hides a common celestial secret… Spica appears to be one star to the naked eye but is in fact two!
The two ellipsoidal stars orbit so close together that their mutual gravity forces them into an egg shape. The two orbit each other every 4 days and are so close that they cannot be resolved individually even through a telescope. Spica is what is known as a spectroscopic binary… where the individual lines in the spectrum show a Doppler shift (Harrington et al., 2009), indicating that there are in fact two stars there orbiting each other (the changes in their orbital motion results in the changes in the spectral lines).
The primary star in Spica has roughly 11 times the Sun’s mass and is much hotter (25,300 K surface temperature, spectral type B1 (Schnerr, 2008)). The secondary star is slightly smaller, at roughly 7 times the Sun’s mass and surface temp ~21,000 K (Schnerr, 2008).
Another red star worth visiting, Antares, is a cool supergiant star in the constellation Scorpius. Antares is a truly colossal star, at over 680 times the radius of the Sun (Ohnaka et al., 2013). Its mass is somewhere between 11 to 14 times that of the Sun, and like other red / orange stars it is very cool in comparison, a spectral type M1 at 3,600 K (Ohnaka et al., 2013). Look for Antares near the Sagittarius region of the Milky Way, stretching across the sky most summer nights, off to the southwest.
If you have a telescope or binoculars, you can view the dazzling globular cluster M4, slightly off to the right of Antares. M4 is a looser star cluster that appears somewhat irregularly shaped and shines with the brightness of over 100,000 stars.
Credits: ESA/Hubble & NASA
This post was fairly long, more so than usual, but that is because there are so many bright and beautiful stars in summer that I just had to share with you! Please scroll down and subscribe to my blog so I can continue to share posts about science and spirituality with you. 😊