Stars, What are they?

Hello Everyone!
Last post, we talked about the life cycle of stars, so this time we are going to talk about what stars actually are and all the 'gritty' details.

Stars are given two different types of magnitudes, absolute and apparent magnitude. Absolute magnitude is the actual brightness of the star while apparent brightness is what the star looks like from a distance (Earth).

Stars are classified by the Hertzsprung-Russell diagram. The Hertzsprung-Russell diagram (HR diagram), named after Ejnar Hertzsprung and Henry Russell, is a graph which astronomers use to help us understand stars. Across the bottom we plot the stars temperature and down the side we plot the stars’ absolute magnitude. By charting stars this way we begin to see at pattern. A star’s temperature and colour depends directly on how big the star is. The bigger a star is the hotter it will be. This is because the stronger gravity of larger stars causes them to burn their fuel more quickly raising the stars temperature. The stronger an objects gravity is, the more power it has to pull its mass inward. This causes the core to be very compact and creates a lot of extra pressure. This extra pressure builds up, raising the temperature of the core. The hotter the core gets, the more of its hydrogen fuel it will burn.

THIS LINK shows an interactive HR diagram.
Answer the following questions:

1. At what age does the example star first enter the main sequence?
2. At the stage of Red Giant, how large (according to the diagram) is the star
3. What is the example given for the White Dwarf?
4. What is the age used for stellar death (in the example)?

The next topic, eclipsing binary stars, is of particular personal interest to me. THIS LINK shows a binary star system simulator. (It's really fun and interesting to play with). 

Set the simulator to the following settings and record you observations about the light curve placed just above the settings. Why do you think this happens?

1. Star 1 with a Mass of 85, radius of 17, and Temperature of 45000K.
2. Star 2 with a Mass of 85, radius of 17 and temperature of 45000K
3. Separation at 60.00
4. Eccentricity at 0.44

The majority of starts are not single stars, they come in pairs of two or more stars (even in star clusters, containing thousands of stars – globular clusters), orbiting around a common gravitational centre and following Kepler’s Laws. Some stars are easy to detect, others require different measures for detection. On the night sky, some stars may appear to be a binary star system, but they are coincidentally placed close to each other.

At first, astronomers thought all stars simply appeared to be double stars, but in 1902 Sir William Herschel discovered that many of the stars that appeared close to each other actually had changed in position relative to each other. Binary stars that can be distinguished through an optical telescope are called visual binaries. An example of an optical binary is the Mizar in the northern hemisphere, which is the second star to the left in the handle of the constellation The Big Dipper, or more precisely: Usae Majoris, The Big Bear.

Figure 1: Mizar using Stellarium

 Mizar (figure 1) is actually a binary system which can be distinguished with the naked eye, and is a fine target for amateur astronomers who wish to test their eyesight. The second component of the star is called Alcor, or Mizar B (figure 2). 

Figure 2: Mizar and Alcor using Stellarium

Later studies have revealed that both stars are in turn binaries, which makes the entire Mizar complex a system of FOUR STARS! (figure 3) 

Figure 3: Mizar Binary using Stellarium

The discovery was made using spectroscopy, which is a method used to detect unseen companions. Binaries discovered with the method are called spectroscopic binaries. When detected, it is relatively easy to calculate the mass of the binary star system, which is more difficult for single stars. The closest star to our sun, located 4.6 light years away is Proxima Centauri, which is actually a system of three stars. Alpha Centauri is the brightest of the three.

Figure 4: A star being pulled into a black hole.

Binary star systems can vary greatly and are very interesting to study. Each component can have an affect on the other partner’s course of life. Systems containing a white dwarf can display a nova, which is an energetic explosion, which can be seen at great distances. Other systems, may contain a neutron star, or even a black hole, which strips off gas from its companion star. The gas is accelerated towards the compact object and heated to more than 1,000,000°K, which is enough to produce X-Rays! (figure 4)

Variable stars are stars that change brightness. The brightness changes of these stars can range from a thousandth of a magnitude to as much as twenty magnitudes over periods of a fraction of a second to years, depending on the type of variable star. Over 150,000 variable stars are known and catalogued, and many thousands more are suspected to be variable.

There are a number of reasons why variable stars change their brightness. Pulsating variables, for example, swell and shrink due to internal forces. An eclipsing binary will dim when it is eclipsed by a faint companion, and then brighten when the occulting star moves out of the way. Some variable stars are actually extremely close pairs of stars, exchanging mass as one star strips the atmosphere from the other.

The different causes of light variation in variable stars provide the impetus for classifying the stars into different categories. Variable stars are classified as either intrinsic, wherein variability is caused by physical changes such as pulsation or eruption in the star or stellar system, or extrinsic, wherein variability is caused by the eclipse of one star by another, the transit of an extra solar planet, or by the effects of stellar rotation.

Figure 5: Parallax

Parallax is the optical illusion that two stationary points change in position relative to each other, due to a difference in position of the person viewing them. The two points in question will be different distances from the observer and the illusion of parallax is cause by the fact that light, follows straight lines (figure 5). When the observer views the nearer point, the line of his vision toward that point is at given angle within the full arc of his vision. For example, let us say that the view straight ahead is zero degrees, and one point, near the observer, is at minus five degrees while a point which is farther away is at minus two degrees. The apparent angular distance between the points is a subjective three degrees to the viewer. If the viewer moves ten metres to his right, the angular direction to the nearer object, as it is on a shorter radius, will change more than the angular direction to the farther object. So, for instance, when the angular direction to the nearer object is at minus ten degrees, the farther object may only have moved to minus three degrees. Now the subjective angular difference in position is seven degrees. The objects appear to have moved relative to each other.

Answers to previous research questions:

1. No, they haven't been proven.
2. Red, yellow, white, blue
3. Nuclear fusion is when two or more lightweight atoms join together to from one heavier nucleus, with any energy released due to the conversion into nuclear energy.
4. Orion Nebula
5. Orion Nebula

Research Questions:

1. Can/Do we use Parallax in our everyday lives?
2. What are some other Binary star systems?
3. Draw your own HR Diagram with the following stars, Bellatrix, Betelgeuse, Sirius, Altair, Maia, Navi and Vega

Q:Why didn't the dog star laugh at a joke?

A: Because it was too Sirius!

Cheers :)

Emily.