Can we really see other stars as true disks? You betcha!

The red supergiant star Betelgeuse in Orion is one of a very few stars for which we have true images. Left photo taken in ultraviolet light by the Hubble Space Telescope in 1995 shows a somewhat misshapen sphere and a ‘hot spot’ below center. Right photo made by the ESO’s Very Large Telescope in near-infrared light reveals the star itself and its extended atmosphere and plume of surrounding gas. Betelgeuse is 640 light years away. Credit: NASA/ESA (left) and ESO

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.

Infrared photo of R Doradus, a red giant variable star in Dorado the Goldfish 370 times larger than the sun. Its brightness and size vary over a period of 339 days as the star expands and contracts like a pumping heart. The star is 178 light years from Earth. Credit: ESO

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.

Direct images of Mira captured by the Hubble in visible light and ultraviolet. The little ‘tail’ at right may be gas flowing from the star blown by strong stellar winds. Credit: NASA/ESA

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.

Layout of the CHARA array on Mt. Wilson. Beams of light from six telescopes are aligned to create images of stars in visible and infrared light. Click to learn more about the facility and see additional photos of stars. Credit: Georgia State University

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!

Altair in Aquila the Eagle – the first image ever taken of a sun-like star using the CHARA array. Altair is only 17 light years away. The temperature contours map out local hot spots on the star. Altair rotates once about every 9 hours, stretching it into an oval shape. Credit: CHARA/GSU

Alderamin, located 49 light years away in the constellation Cepheus, is another fast rotator. Hot spots are also seen. Credit: CHARA/GSU

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.

The giant star Zeta Andromedae located 181 light years away in Andromeda. The bright regions are large starspots, similar to our own sun’s sunspots. Credit: CHARA/GSU

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.

This is a model of the star Regulus based on data obtained with the CHARA array. Regulus not only has an extreme shape but it’s large too. It spins at the rate of 709,000 mph. Credit: Wenjin Huang/CHARA

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.

Catch a glimpse of the world’s end in Mira’s burning eye

Seven billion years from now the sun will swell up to engulf the planets Mercury and Venus due to changes in how it burns fuel in its core. Credit: B. Jacobs

Big changes are coming for the sun. Hydrogen makes up 3/4 of the sun’s mass, an element it “burns” via nuclear fusion to create the light and energy that stream to Earth every day. As with any fire, there’s leftover ash. For the sun that means helium, the byproduct of the fusion of hydrogen atoms. In my wood stove, I remove the ash every few weeks to make more room for wood, but in the sun, the heavier, denser helium ash sinks to its center and heats up under the pressure of gravity.

Some 7 billion years from now, when all the core hydrogen’s used up, not only will the ash be extremely hot (think of glowing embers), but hydrogen will begin to burn in a shell around the core. The energy from these two processes creates lots of outward pressure that will cause the sun’s outer layers to expand into space. Since they’ll be farther from the hot core, that sun’s surface will also cool and redden. And just so you know, we’re talking major expansion — the planets Mercury and Venus will be engulfed by the newly-bloated sun. A humanoid looking skyward 7 billion years hence will see a brilliant, orange-colored ball of light spanning nearly half the sky while standing on a baked, waterless Earth. Beautiful in its way but grim.

On that faraway day, our sun will have evolved into what astronomers call a red giant. More fun will be in store as we move further forward in time. Eventually the helium ash will ignite and the sun will shrink again, followed by another expansion and the casting off of its outer layers to reveal a tiny, planet-sized core called a white dwarf. For more on the sun’s evolution, you might like to check out this site.

This video, compiled using photos taken by a Paris Observatory team of astronomers, shows the red giant star Chi Cygni (Kye-SIG-nye) in the Northern Cross. Its light varies regularly with and expansions and contraction cycle of 408 days.

The current sun is stable and reliable because the inward pull of gravity balances the outward pressure of heat production from nuclear fusion. Not so with red giants where the two forces are in a constant tug of war with one other. When gravity wins, the star contracts and heats up. Fine and good, but the extra heat is now too much for the star to handle. To compensate, the outer layers expand and cool. Gravity soon gets its way again and initiates another contraction. The stars pulsate with periods ranging from 80 to 1,000 days and vary in brightness with every contraction and expansion.

To find Mira, face east around 11 p.m. and locate brilliant Jupiter. Now make a fist and extend your arm to the sky. Menkar is a little more than one fist below Jupiter, and Mira is one fist directly right of Menkar. Menkar, Mira and Deneb Kaitos all lie on a straight line. Created with Stellarium

If you want to see what the sun will look like billions of years from now, look no further than Mira the Wonderful, one of the easiest-to-follow red giants in the sky. Mira’s in the constellation Cetus the Whale and bright and easy to see right now. I’ve been keeping tabs on the star since July and was delighted a few nights ago to discover it had brightened to magnitude 2.4, similar to the stars in the Big Dipper. What’s more, Jupiter’s nearby, making it especially easy to find.

Mira is big enough to show a shape as photographed by the Hubble Space Telescope in 1997. The irregular shape may either be due to the expansion-contraction cycle or surface features. Credit: NASA/ESA

Mira typically varies in brightness from about 3rd magnitude to 10th over a period of 331 days. Late September finds the star contracting and brightening to maximum. It will remain bright for some weeks before expanding and fading again.

The discovery of Mira’s variability was made in 1596 by German theologian David Fabricius who first thought he was seeing a “new star” or nova. Thousands are known and studied today by both professional and amateur astronomers. As a variable star observer, I regularly observe Mira-type stars and submit my observations to the world headquarters of variable stars, the AAVSO. There, astronomers use the information to further understand their behavior.

Mira’s temperature varies from about 3000 degrees Fahrenheit when fully puffed out to 4200 when sucked in, and the star’s diameter varies by 20% over its 11-month cycle. At largest, it’s 330 times larger than the sun.

This picture, taken last night, shows Jupiter and helpful stars you can use to find our featured star Mira. Photo: Bob King

Once you find Mira, keep an eye on it in the coming weeks and months. By early winter, the star will have “disappeared” from the constellation. Of course, it’ll still be there, but you’ll need binoculars or a telescope to see it. Speaking of which, take a look at it in binoculars and you’ll see that Mira has a warm, yellow-orange color. As it fades, the color will become slightly redder.

Mira presents an excellent opportunity to get to know a pulsating red giant and imagine the future sun. Check it out the next clear night.

Meet Miss Wonderful

Last night, Comet Hartley 2 pulled to within spitting distance of the rich pair of clusters, NGC 869 (left), and NGC 884. Details: 200mm lens at f/2.8, ISO 800, 2-minute exposure on a tracking mount. Photo: Bob King

Yesterday night was splendid for comet photography. Comet Hartley 2 slowly edged in toward the Double Cluster making for a picturesque portrait of two faraway star clusters and one fuzzy green visitor from the inner solar system. I hope you had the opportunity to see the comet yourself last night. Tonight Hartley 2 will still be in the neighborhood but on the other side or east of the clusters.

This map shows the sprawling constellation Cetus around 11 o'clock in early October. Deneb Kaitos is two outstretched fists below Jupiter. Mira is level with and about three fists directly to the left of Deneb Kaitos. Mira is currently a bit brighter than Gamma (magnitude 3.5) and a little dimmer than Alpha (2.7). Created with Stellarium

As the ceaseless rotation of the Earth put the western stars to bed, Cetus the Whale rose to prominence in the southeastern sky. Fittingly, this whale of a constellation takes up a large chunk of sky. I only wish it had a few more bright stars to make its gangly easier to see; second magnitude Deneb Kaitos is the best it can muster. But what Cetus lacks in luster, it more than makes up for in Mira. This oddball star, located a couple “fingers” to the right or west of Alpha Ceti, is “missing” most of the year. You would have searched in vain for it during August and early September – unless you had a telescope. Mira spends much of the year below naked brightness and only a few months above. We’re lucky that it’s at peak brightness right now and plainly visible without optical aid.

Mira photographed in visible light by the Hubble Space Telescope. It bloated, tenuous atmosphere distorts the shape of the star, which is 420 light years from Earth. Credit: NASA/ESA

Mira is the prototype of a class of stars called long-period variables (LPVs). Though similar in mass to our sun, they’re far larger and pulsate – physically expanding and shrinking – over regular intervals. Mira was discovered by David Fabricious in 1596. Prior to that time, no one knew a star could appear and disappear, which is why it was eventually named after the Latin word for “wonderful”. Over the course of 11 months, Mira ranges from ninth magnitude (visible only in a small telescope) to as bright as second (similar to the Big Dipper stars). This month it hovers near its current maximum brightness of around third magnitude or one level fainter than the Dipper stars.

Mira oscillates from 400 to 700 times the size of the sun over a period of 332 days. While it might seem odd at first, the star is brightest when smallest or most condensed. As it shrinks, the amount of energy radiating from a particular patch on the star’s surface increases, making it overall brighter and hotter. That’s the stage Mira’s in this month. Just the opposite happens when the star expands to its maximum diameter. That same energy is then spread over a much larger surface area and the star’s surface radiates less heat and light. As Mira cools from around 5000 degrees Fahrenheit to 3000, a compound found in sunscreen, titanium dioxide, condenses in its atmosphere and does the same thing it does on your body – block UV light. It not only removes UV radiation but some visible light as well, resulting in additional dimming.

Mira is a red giant star on its way to becoming a white dwarf. It also has a white dwarf companion (right) which undoubtedly evolved from a similar red giant. Credit: David Aguilar, Harvard-Smithsonian Center for Astrophysics

When Mira’s huge, it can barely hold onto its vast atmosphere. Powerful winds blast away 10 million times more material into space each year than our own sun’s solar wind. Mira’s evaporating! Soon its atmosphere will expand away into space as a beautiful planetary nebula illuminated by Mira’s core, now reduced to a white dwarf, a blazingly-hot but tiny star the size of the Earth. Take a good look at yourself. While we’re mostly made of water, we also contain plenty of carbon. Elements like carbon are cooked up in the core of stars like Mira and then released into space by those strong stellar winds. Those elements eventually found their way right into our very being. If that ain’t a whole lotta wonderful, I don’t know what is.

The sun’s evolutionary path will parallel that of Mira’s, but those days are still several billions of years in the future. I hope you’ll take the time to find our “wonderful star” this fall. If you spot it now, you’ll have the pleasure of following its inevitable fading in the coming months. Or perhaps it will even get brighter – LPVs aren’t 100% predictable. During one cycle, Mira became nearly as bright as first magnitude Aldebaran; I’ve seen it hit second. Can you see its warm hue with your naked eye? I think I can. Binoculars show it better.

For the more adventurous, try making careful estimates of Mira’s changing light by checking out the American Association of Variable Star Observers website. In the ‘Star Finder’ window at right, type in ‘Mira’ and click ‘Create a finder chart’. When you’re presented with the default chart, click on ‘Change chart parameters and replot’, select ‘A’ in the Legacy chart scale window and click ‘Plot chart’. Star brightnesses on the charts are given to the tenth of a magnitude with the decimal omitted.