Part 10: Basic Properties of Stars
John P. Pratt
- The brightest stars have names, generally of Arabic origin.
- They also are designated by the Greek letters, like alpha, beta, gamma, followed by the name of
- The Greek letters are usually in order of brightness, so brightest is usually alpha.
- For example, the star Sirius is alpha Canis Majoris (brightest in Big Dog, Canes Major).
- You should memorize all 15 of the brightest stars visible from your location.
Looking at Stars
- All stars are so far away that they appear thousands of times smaller than a pinpoint.
- They look a lot bigger than they are because the light triggers one cell in your eye, which is the smallest thing
you can see.
- Stars twinkle because as the narrow beam of light traverse the air, it can bend and miss your eye.
- If you are using a telescope, try picking a night when the stars do not twinkle much. Called good "seeing."
- You can see dim stars by looking several degrees away from them, using peripheral vision, because
the place in your eye that focuses best is not the most sensitive to light.
- Several stars are bright enough to see their actual colors, especially as red, orange, yellow, white and blue.
Photographs of Stars
- Bright stars make big images in photos, but that image is not a picture of the star.
- A big star image is formed because the light gets smeared out in a circle.
- The rays sometimes seen coming from stars in photos are images of the secondary mirror supports.
- The rings and disks around stars images are caused by diffraction.
- Astronomers do not use photographs much to measure star properties.
- Some colors in photographs are not accurate, but a caused by film failures. The Orion Nebula
looks red in photos (from Hydrogen's H-alpha line) but looks green to the eye (from Oxygen).
- The light year (ly) is the distance light travels in one year, being about six million million miles.
- The nearest star is over four ly away. The light from the sun takes about 8 minutes to reach earth.
- Astronomers also use the par sec (pc), which is the distance of a star with a parallax of one arc second.
- No star is as close as one par sec, but is close to that, being 1.3 pc. Stars are about 1 pc apart in our stellar neighborhood.
- 1 pc = 3.26 ly.
Apparent Brightness of Stars
- Apparent brightness is how bright a star appears to be, whether or not it is a nearby star, or extremely distant.
- We still use the Greek system of apparent magnitudes.
- The brightest stars were said to be first magnitude.
- The dimmest stars visible in a dark sky far from a city are sixth magnitude.
- The dimmest stars visible in a city are often only third magnitude.
- There are fifteen first magnitude stars visible from the U.S. You should know their names and locations.
- All of the stars in the Big Dipper are second magnitude stars.
- The four stars in the bowl of the Little Dipper are of 2nd, 3rd, 4th, and 5th magnitude, so they are a good
measuring stick for comparisons.
- Modern astronomers define magnitudes so that a difference of five magnitudes is a factor of 100 in
brightness. That means that some very bright stars have 0 magnitude, or even negative. Sirius has magnitude -1.
- Venus has a magnitude of -4, the moon is about -12, and the sun is -26.
Measuring Properties of Stars
- Absolute magnitude is defined to be what the apparent magnitude would be at a distance of 10 pc.
- It measures the "absolute" brightness of a star, that is, how much light it is really emitting.
- The absolute magnitude of the sun as about +5.
- The absolute magnitude of Rigel is about -7, meaning it is really incredibly bright.
- Many of the stars visible in the sky are the bright beacons. There are many closer stars too dim to see.
- Absolute magnitude usually means the brightness at all wavelengths, also called absolute bolometric magnitude.
- If one wants to refer only to wavelengths of visible light, then it is the absolute visual magnitude.
- A sprectrograph spreads the stars light into all of its component colors onto photographic film.
- From stellar spectra, we can determine temperature, composition, pressures, rotational velocities, etc.
- Most spectra have many absorption lines causes by cooler gases which absorb the light.
- Stars are classified by their overall spectral characteristics.
- The classes are O, B, A, F, G, K, M ("Oh, be a fine girl, kiss me.")
- Those classes correspond to blue (O,B), white (A, F), yellow (G), orange (K) and red (M) stars.
Spectral Class Facts Worth Memorizing
||Temperature (K)||Absorption Lines
|Blue||O||30,000||Ionized Helium||Orion's Belt|
|Blue-White||A||10,000||Hydrogen (thick lines)||Sirius, Rigel|
|White||F||7,000||Hydrogen (thin lines)||Procyon|
|Yellow||G||5,500||Ionized Metals||Sun, Capella|
The Doppler Shift
- Light eminating from a source moving away from you is red-shifted, that is, the lines are moved toward red.
- Light from a source moving toward you is blue-shifted.
- The Doppler shift is used to measure a stars radial velocity, the velocity toward or away from us.
- A star's rotation rate can be measured by broadening of spectral lines (half of star approaching, half receding).
Measuring Distances to Stars
- Parallax (seeing nearby stars move relative to distant stars) can only be used for the nearest stars, within 100 pc.
- Remember the parallax of even the very nearest star is less than one second of arc, which is extremely small.
- There are other ways to calculate distances to more distant stars when other things are known, such as the
Luminosity and the Stefan-Boltzman Law
- The luminosity of a star is another unit to refer to the absolute magnitude of a star (at all wavelengths).
- The unit is the luminosity of the sun. That is, the luminosity of the sun is 1.
- The Stefan-Boltzman law is that the luminosity of every square meter of a star increases as the fourth power
of the temperature.
- That means that Luminosity is proportional to AT4, where A is the surface area, and T is the
- For the nearest stars where the distance is known, one can calculate the luminosity because the apparent
brightness decreases with the square of the distance.
- There are other ways to determine the luminosity of certain stars, as will be discussed later.
- There are only a very few stars large enough to measure the diameters directly.
- Most stars have their diameters calculated from their temperature and luminosity, using the
- Masses for stars can be measured using binary stars (two stars in orbit around their center of mass).
- The sum of the masses is known from the orbital period using Kepler's laws.
- The ratio of the masses is known from measuring its distance from the center of mass.
- From the ratio and sum of the masses, the mass of each can be calculated.
- Stars with nearly identical spectra usually have nearly identical masses.