Star of the Week: Altair

Returning once more to the Summer Triangle, I'd like to introduce readers to Altair.

Altair (false color) imaged by MIRC at the CHARA Array on Mt. Wilson
Credit: John Monnier (University of Michigan)

Altair has the distinction of being one of the few stars (besides the Sun) which has been directly imaged. What you're looking at there, in false color, is a reconstructed image of Altair from observations with the CHARA (Center for High Angular Resolution Astronomy) Array, an array of 1m telescopes that uses the combined light from each instrument to collect high resolution observations of astronomical objects. In a sense it acts like a much larger telescope and I'll explain CHARA here in an upcoming post.

Altair is the brightest star in Aquila and is classified as A7V star. It is 16.7 ly from Earth, a neighbor and a close one at that on a galactic scale.  If you look at the picture above, something that should be noticeable is that Altair is not spherical. This is because it rotates at a high speed. Since stars are made of gas, a rapidly rotating star will have it's shape altered by the motion, bulging outward. You might also notice that there are light and dark spots. This indicates areas where the star is different luminosities (white being more luminous). This is because luminosity and temperature are related to surface gravity, which in turn depends on mass and radius. In Altair, the surface gravity is lower at the radius, meaning there is lower gas pressure, cooling the gas there and decreasing luminosity.

Updates for the coming months

A 'rite of passage' for many graduate students are the qualifying (sometimes called comprehensive) exams they must pass as part of their program. The details of these exams vary by department but the universal trait is that they are among the most difficult moments in graduate school. This coming semester I have my second (and most comprehensive) round of qualifying exams.

Because of the exams, I might not post all that much, but I do intend on posting more than I did last semester (not at all!) because the exams cover all of astronomy and what better way to review all of astronomy than by sharing bits of it with others via introductions to various stars? So, although the Summer Triangle is long gone, there will soon be the promised introduction to Altair.

Coming Soon: Altair, one of the only stars apart from the Sun which has been directly imaged.

Tomorrow or more likely Monday Tueday, I'll return to the Star of the Week. This time I'll be writing about Altair, another member of the Summer Triangle. This star is of special interest because it has been directly imaged. This means that there is a picture of the surface of the star, not just some point in the sky. This was done with an instrument operated by my university which I hope to use in my own research! It turns out, moreover, that Altair is not a sphere. It spins so fast that it bulges.

Star of the Week: Deneb

If I had a pet swan, and I'm not sure why I'd ever have one because swans are rather mean creatures, I'd have to name him or her "Deneb". This isn't because "Deneb" means "swan". The name comes from the Arabic word for "tail". Rather it is because Deneb is the brightest star in the constellation of Cygnus, the swan, and is so closely related with swans in my mind that I cannot help imagining one when I hear the name.

This  is why I would never have a pet swan
Credit: Flickr user: ellenm1

Not only one of the brightest stars in the sky, Deneb is one of the most luminous stars as well. The difference between luminous and brightness is explained by distance. Astronomers measure how bright something seems with two numbers: apparent magnitude, or how bright it appears on Earth, and absolute magnitude, or how bright it would appear at a distance of 10 parsecs (32.6 light years). Because stars are at varying distances, using absolute magnitude allows them to compare the stars.

What makes Deneb so luminous? It is very hot and very big. Deneb is a type A supergiant (I will explain what this means in a separate post) with a temperature of 8400 K. Although it is much larger than the sun, it is only 20 times as massive, meaning that actually it is rather tenuous, as is common for large stars. Like Polaris, Deneb is a variable. In fact, Deneb lends its name to a class of variables, the Alpha Cygni variables. In these stars, one part of the star might be expanding whilst another is contracting!

The Summer Triangle

One of my favorite asterisms is the Summer Triangle.

But wait! One might ask, '"What's an asterism?"    

An asterism is what is most people mean when they say "constellation". It is pattern in the sky made by stars. A constellation, in the astronomical usage of the word, is a region of the sky. Constellations contain asterisms but also galaxies, nebulae, star clusters, and much more.

The Summer Triangle is an asterism comprised of three bright stars
Credit: Flicker user Charles de Mille-Isles

The Summer Triangle is an asterism comprised of three bright stars, each in three different constellations: Deneb, in Cygnus; Vega, in Lyra, and Altair, in Aquila. Although it is visible in the spring and autumn, in the Northern Hemisphere, these stars are visible near the zenith (the region of the sky directly overhead) throughout the night during the summer, hence the name. My affection for the Summer Triangle lies in memories of evenings spent with my family looking up at the stars in my front yard. Even in a suburban neighborhood littered (I choose that word carefully) with torch-like security lights, the Summer Triangle shines brightly in the sky. 

For the next three weeks, I plan on featuring each of the stars in the Summer Triangle. I'll begin this week with Deneb, located in Cygnus, "The Swan", the brightest stars of which form another asterism, "The Northern Cross". In fact, I can hardly think the name "Deneb" without the image of a swan popping in my mind. 

Polaris Part Two

How 'constant' is the northern star? Last week, I noted that the role of the northern star changes over the millennia  due to the precession of the equinoxes. But even the star we call the north star today is not so constant! Its brightness varies!

Like almost half of stars, Polaris belongs to a multiple star system. Although such systems can have more than two stars they are generally referred to as 'binary stars'.  What we see with our eyes as one star, a telescope would reveal as two (Polaris Aa and Polaris B) and a better telescope would reveal as three (Polaris Ab). There are two other stars, more widely separated from the other three, that were once thought to be part of the system, but X-Ray observations with the Chandra Space Telescope suggest these stars are older than the other three and therefore did not form with them.

Astronomers, rather than coming up with new names for each star in a binary system, keep the same name for every star in the system and use capital letters (A B C ...) to distinguish each star. A lowercase letter denotes an object which orbits or is paired  up (so orbiting a common center of mass) with only one of those objects. In this case, Polaris Aa and Polaris Ab orbit together, and Polaris B is paired with that pair of stars. The two non-related companions are still called Polaris C and D, even though they are no longer thought to be part of the system.

Images of Polaris A, Ab, and B via the Hubble Space Telescope
Credit  listed at bottom of photo original image found at hubblesite.org

Of the three stars, it is Polaris A, the star we see with our naked eyes that is the most interesting. It is a Cepheid Variable. These stars are of immense importance in modern astronomy because the amount of time it takes for one to undergo a full cycle of dimming and brightening (period) is related their luminosity. Because astronomers use luminosity as means of determining distance, this means that by observing the period of a Cepheid astronomers can determine the distance to a star, even if it is too far away to accurately determine its luminosity. In this way, we are able to determine the distance to other galaxies.
 Polaris is of particular importance because it is the closest Cepheid to Earth and because it is a binary. That Polaris is close is important because it means we can determine its distance by other methods, such as parallax, which allows us to determine it's luminosity independently of period  and fine tune the relationship between period and luminosity.  Moreover, because it is a binary, it is possible to determine the mass of the star, which helps astronomers work on the theory of why Cepheids pulse.

In later posts I'll detail more about the nature of Cepheids and describe the entire Cosmic Distance Ladder. For now, remember that Polaris, is not nearly so constant as Shakespeare had Caesar suppose!

Star of the Week: Polaris

One of the regular features I'd like to have on this blog is 'Star of the Week', in which I introduce readers to a star in the sky. It's likely that along with this, I'll talk a bit about the constellation in which the star is located,  but I'm considering having a 'constellation of the month'.

Appropriately, the first 'Star of the Week' is Polaris--the North Star.

Polaris is not an especially exceptional star. It is the brightest star in Ursa Minor (hence it has the name Alpha Ursae Minoris) but with an apparent magnitude of around 2 it is the 46th brightest of stars (in both hemispheres). Were it not for it's location, Polaris would be no more familiar than Alphard, the brightest star in the constellation Hydra, which has a similar apparent magnitude. But, what a location it has, for Polaris has a distinguished position close to celestial north pole. This means that to observers in the northern hemisphere, Polaris wobbles in place ever so slightly while all the other stars trace circles around it. It also means that at night, one can find north by finding the appropriately nicknamed North Star. Moreover, should anyone ask you to find Polaris during the day, although you cannot see it, you can confidently point north.

Stars traverse the sky at night, but not Polaris, which sits
like a king in the center of a court.
Photo : Ashley Dace under Creative Commons Attribution Share-alike

The fact that there is a star near the celestial north pole is a happy coincidence of both time and space. At the moment, Polaris happens to be located above the axis of Earth's rotation. However, the orientation of Earth's axis of rotation wobbles, a process called the precession of the equinoxes. As a result Polaris was not the North Star for the Ancient Greeks and in the coming centuries, it will be drift further and further away from north.

One can find Polaris by using a simple trick involving one of the most easily identifiable constellations, Ursa Major (also known as the Big Dipper). Using the two stars on the leading edge of the dipper section (the two stars away from the handle) follow up from the bottom star. The next bright star you find is Polaris. One can also locate the star at the edge (back end) of the handle part of the Littler Dipper (Ursa Minor) but because that constellation has many dim stars, it is difficult to find it in places with even moderate light pollution.

Besides finding north, one can also use Polaris to get a rough estimate of latitude or distance above the equator: the height of the star roughly corresponds to one's latitude. At the North Pole, it is overhead, whereas at the Equator it is skimming the horizon.

In a post later this week, I'll detail information about the nature of the star. Sneak Peak: What we call the North Star actually more than one star!