Stars can be huge – Betelgeuse in Orion is 1,500 times the size of the sun – but they’re so far away that even in the largest telescopes they’re unresolved pinpricks of light. Only the sun shows a substantial disk. But if you placed it at the distance of even the nearest star system, Alpha Centauri, it would shrink to a point just like all the other stars we see in the sky at night.
Distance cuts any star down to size. The Hubble Space Telescope orbits 354 miles (570 km) above the Earth, high above the turbulent atmosphere and can see a supergiant star like Betelgeuse as an exceedingly small but actual disk at the limit of its resolving power.
The same way we measure the angular size of the moon and sun against the sky, astronomers measure how big a star appears from Earth. Half a degree – the diameter of the full moon – equals 30 arc minutes or 1,800 arc seconds. Betelgeuse spans a mere 0.045 arc seconds or 40,000 times smaller than the moon! And it’s one of the largest stars up there. It was also the first star beyond the sun to be photographed as a disk. We’re not looking at overexposed images here – these are pictures of the actual stars.
R Doradus is another biggie in the southern constellation Dorado. A red giant star whose brightness and size change in a repeated cycle of expansion and contraction, R has the largest apparent size after the sun. That’s not saying much – at 0.057 arc seconds, it’s miniscule. Ditto for beautiful Mira, another red giant variable star, in Cetus the Whale.
Like many red giants, Mira loses a lot of material to space from powerful stellar winds generated as the star expands and contracts. Oh, but there’s still plenty left. Put in place of the sun, this red balloon of a star would reach all the way out to the asteroid belt.
To get pictures of more and smaller stars, astronomers need bigger telescopes.That takes time and money, though several are in the works like the 1,547-inch (39.3-meter) European Extremely Large Telescope. In the meantime, professionals use a technique called interferometry that combines the light coming from several telescopes to yield a resolution equivalent to a telescope as large as the distance between them.
One of the premier facilities for measurement and imaging of stars using interferometry is Georgia State University’s Center for High Angular Resolution Astronomy (CHARA) that uses an array of six telescopes located on Mt. Wilson in California. Each of the 39-inch (1-meter) diameter telescopes are positioned across a mountaintop to yield the equivalent of one giant telescope over 1,000 feet across!
Light from the individual telescopes is conveyed through vacuum tubes to a central facility where the six beams are precisely aligned and combined together. Although this synthetic telescope lacks the light-gathering ability of a real ’1,000-footer’, it does have the equivalent resolving power. CHARA can resolve details as small as 200 micro-arcseconds, equivalent to the angular size of a nickel seen from a distance of 10,000 miles.That means it can drill down to image smaller stars and get clearer views of larger ones.
Using the array, astronomers have successfully created images of Altair in Aquila, Zeta Andromedae, Rasalhague in Hercules and others.
One of the most amazing stars imaged is Regulus, Leo’s brightest luminary.
Regulus rotates once every 16 hours compared to the sun’s average spin of 25 days. Rapid spinning has caused the star to spread out at its equator into a long oval shape called an oblate spheroid.
Another effect that CHARA sees in rapidly rotating stars like Regulus and Altair is ‘gravity darkening’. Since Regulus spins so rapidly it experiences more centrifugal force at its equator compared to the poles. The pull of gravity is less there, causing the gas to be less dense and cooler (darker). Stronger gravity at the poles compresses the gas more and causes it to brighten.
While my examples aren’t exhaustive, they provide us with a new dimension on just what we’re looking at when we stare up at those thousands of sparkly lights in the night sky.