The Cosmic Distance Ladder II, Stars as Standard Candles - Page 2

Articles in this Topic Discussions in this Topic

Page 2

Dec 1, 1998

was use of Cepheids by Edwin Hubble to determine the distances to nearby galaxies. When he measured the velocity at which these galaxies were receding (the redshift) from us, he noticed that the velocity was proportional to the distance, and hence demonstrated that the Universe is expanding. Since then, a major effort in astronomy has been to measure the constant of proportionality called H_₀, the Hubble Constant in the distance-redshift relation, and only now, 70 years later, have we begun to get a firm handle on the subject.

Stellar Evolution: Stars of given chemical composition and mass behave

nearly identically to each other, and evolve in nearly identical ways. Thus, if one can assess the composition and age of a star, the star becomes a good standard candle. In a particular stage of evolution, stars enter the Red Giant stage. Remarkably, stars in this stage, despite having different mass, typically have quite similar brightness and temperature. They form what is known as the the red clump, so called because of the crowded appearance they present on a plot of luminosity versus temperature. As long as chemical composition doesn't vary radically, these red clump stars form very good standard candles. Recently, the Hipparcos satellite (see the previous article) obtained excellent parallax distances to 100's of these stars and firmed up their absolute luminosities. Currently, these standard candles are being used to confirm the Cepheid distance to the Large Magellanic Cloud, a critical stepping stone to the extra-galactic distance scale.

Eclipsing binary stars: Double stars which eclipse each other exhibit an obvious orbital period, even if the two stars cannot be resolved. During eclipse the total light is less. By measuring the spectral Doppler shift of the light from the stars, one knows the speed at which the stars are orbiting. Knowing the speed and the period allows determination of the distance between the stars. Knowing speed and distance allows one to use Kepler's laws to determine the mass of the stars. One may then return to the stellar evolution method to estimate the light that should be emitted from a star of given mass and composition to create another standard candle. This method is somewhat difficult because of the faintness of most double stars outside the galaxy.

All of these methods rely solely and heavily on parallax distances to nearby stars, but are necessary to step our way out of the galaxy in determination of the cosmic distance scale. Next time, I will discuss major efforts currently underway to stepping up the distance ladder into the realm of the galaxies.

Go To Page: 1 2

The copyright of the article The Cosmic Distance Ladder II, Stars as Standard Candles - Page 2 in Astronomical Events is owned by . Permission to republish The Cosmic Distance Ladder II, Stars as Standard Candles - Page 2 in print or online must be granted by the author in writing.