Gamma Cas and Friends (2009 Dec 24)

[APOD Robot]

Gamma Cas and Friends

Explanation: Gamma Cassiopeiae shines high in northern autumn evening skies. The brightest spiky star in this rich and colorful Milky Way starfield, bluish Gamma Cas marks the central peak in the W-shaped constellation Cassiopeia. A hot, variable, and rapidly rotating star about 600 light-years distant, Gamma Cas also ionizes surrounding interstellar material, including the wispy IC 63 (left) and IC 59 emission and reflection nebulae. The two faint nebulae are physically close to Gamma Cas, separated from the star by only a few light-years. This well-composed, wide-field view of the region spans almost 2 degrees on the sky.

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[neufer]

<<Gamma Cassiopeiae (γ Cas / γ Cassiopeiae) is an eruptive variable star, whose brightness changes irregularly between +2.20 mag and +3.40 mag. It is the prototype of the Gamma Cassiopeiae variable stars. Although it is a fairly bright star, it has no traditional Arabic or Latin name. In Chinese, however, it has the name Tsih, meaning "the whip". It is located at the center of the distinctive "W" shape that forms the Cassiopeia constellation. The star was used as an easily identifiable navigational reference point during space missions.

The apparent magnitude of this star was +2.2 in 1937, +3.4 in 1940, +2.9 in 1949, +2.7 in 1965 and now it is +2.15. At maximum intensity, γ Cassiopeiae outshines both α Cassiopeiae (magnitude +2.25) and β Cassiopeiae (magnitude +2.3). This is a rapidly spinning star that bulges outward along the equator. When combined with the high luminosity, the result is mass loss that forms a disk around the star. The emissions and brightness variations are apparently caused by this "decretion" disk.>>

<<What distinguishes Be stars from normal stars on or near the main sequence? One thing is their rapid rotation -- up to 450 km/sec at their equator. [Rotational velocity of the sun ~ 2 km/sec at the equator; solar orbital velocity ~ 437 km/sec at the equator.] This reduces the effective gravity at the equator of the star. The strong radiation of B stars (they are thousands of times more luminous than the sun) produces a "stellar wind" which, in Be stars, is focussed into an equatorial disc. The shell stars are mostly Be stars in which we see the disc edge-on: gases in the disc produce the deep, narrow absorption lines. Be stars are also a major contributor to "galactic ecology" -- the process by which stars lose mass, which becomes part of the raw material from which new stars and planets form. With modern optical interferometers -- two or more telescopes which image an object simultaneously -- it is possible to "see" these discs. Another possible factor in "the Be phenomenon" may be a magnetic field, but no field has yet been observed in most Be stars.

Be stars vary in brightness, and spectrum, on several different timescales. There are variations on time scales of weeks to decades, which are connected with the formation and dispersal of the disc. These variations may be cyclic in nature; according to one theory, this is due to a spiral wave which slowly circulates around the disc. There are variations on time scales of days to weeks which are often connected with the binary motion of some of these stars; one example -- CX Dra -- is described below. Finally, there are variations on time scales of 0.3 to 2 days, which are due to non-radial pulsation, or perhaps rotation. These variations, which occur on or near the surface of the star, may be connected with the formation of the disc around the star. AAVSO photoelectric observations are used primarily to study the slow variations, but a few photoelectric photometrists have participated in intensive "campaigns" to study the rapid variations.>>

[smitty]

How do we determine the rotation rate of stars (or even the fact that they rotate at all)? I can understand how we might do this for our sun by observing sun spots or other solar phenomena having a temporal duration, but how do we do it for more distant stars?

[neufer]

How do we determine the rotation rate of stars (or even the fact that they rotate at all)? I can understand how we might do this for our sun by observing sun spots and other solar phenomena having a temporal duration, but how do we do it for more distant stars?

Other stars also have sun spots or other localized solar phenomena.

These phenomena produce sharp UV & X-ray absorption line features
with strong periodic amplitude & (Doppler shift) frequency variations.

http://www.iop.org/EJ/article/0004-637X ... 39019.html

[Chris Peterson]

How do we determine the rotation rate of stars (or even the fact that they rotate at all)? I can understand how we might do this for our sun by observing sun spots or other solar phenomena having a temporal duration, but how do we do it for more distant stars?

There are a variety of methods. Certainly, many other stars have starspots like our Sun's, and these show up photometrically as the star rotates. (One of my own areas of research involves photometric observation of rapid rotators.) Stars can also have hot spots that come into view regularly with rotation. Depending on the class of star, rotational structure may also show up in x-ray or radio measurements.

In the case of stars like Gamma Cas, the rotation rate is so high that the star is significantly flattened, and this disc-like shape can sometimes be imaged directly with interferometric telescopes, and can have its orientation and size estimated spectroscopically.

Some stars don't show measurable rotation, but are assumed to rotate in a certain rate range simply because they fall into a stellar class which has other members with accurately measured rates in that same range.

[smitty]

Thank you for your helpful replies to my earlier question. Now, other questions which spring to mind are why do stars rotate at all, and why do some rotate faster than others? Seems quite odd. What spins them up?

[Chris Peterson]

Thank you for your helpful replies to my earlier question. Now, other questions which spring to mind are why do stars rotate at all, and why do some rotate faster than others? Seems quite odd. What spins them up?

Stars rotate because of the conservation of angular momentum. The cloud of material that collapses to form the star always has some net angular momentum (it doesn't absolutely have to, but there is only one case of zero, and an infinite number of cases where the momentum is positive or negative; realistically zero angular momentum is a vanishingly small possibility). In the same way a figure skater speeds up when she pulls in her arms, a star speeds up as the material collapses into a smaller radius. The amount of spin for a new star will depend on the starting angular momentum of the cloud and the degree of collapse experienced.

This doesn't just apply to stars; it is why galaxies and galaxy clusters rotate, it is why planets orbit stars, why they rotate on their axes, why satellites rotate, and in general why pretty much everything in the Universe rotates.

[smitty]

An excellent, logical, understandable answer to the question of rotation . . . thank you! It makes perfect sense when seen this way. This also apparently explains why some stars (is it neutron stars?) rotate at unimaginably/unbelievably high rates . . . many rpm's?

[Chris Peterson]

This also apparently explains why some stars (is it neutron stars?) rotate at unimaginably/unbelievably high rates . . . many rpm's?

Exactly. You have a star which contracts in size by five or six orders of magnitude; thus a "normal" rotation rate of days or weeks is reduced to seconds or less in order to conserve the original angular momentum (less whatever was transferred to any ejected material).