Basic Properties of Stars

Stellar Position and Distances Astronomers use two units of measure for long distances. A light-year (ly) is the distance that light travels in one year, equal to 9.461 ×1012 km. A parsec (pc) is equal to 3.26 ly

Stellar Position and Distances- Parallax To estimate the distance of stars from Earth, astronomers make use of the fact that nearby stars shift in position as observed from Earth. Parallax is the apparent shift in position of an object caused by the motion of the observer.

Stellar Position and Distances - Parallax The distance to a star, up to 500 pc using the latest technology, can be estimated from its parallax shift.

Basic Structure of Stars The mass and the composition of a star determine nearly all its other properties. Hydrostatic equilibrium is the balance between gravity squeezing inward and pressure from nuclear fusion and radiation pushing outward. This balance, which is governed by the mass of a star, must hold for any stable star; otherwise, the star would expand or contract.

Basic Structure of Stars Fusion Inside a star, the density and temperature increase toward the center, where energy is generated by nuclear fusion. Stars on the main sequence all produce energy by fusing hydrogen into helium, as the Sun does. Stars that are not on the main sequence either fuse different elements in their cores or do not undergo fusion at all.

Basic Structure of Stars Fusion Fusion reactions involving elements other than hydrogen can produce heavier elements, but few heavier than iron. The energy produced according to the equation E = mc2 stabilizes a star by producing the pressure needed to counteract gravity.

Basic Properties of Stars Magnitude One of the most basic observable properties of a star is how bright it appears.

Basic Properties of Stars Apparent Magnitude Apparent magnitude is based on the ancient Greek system of classification which rates how bright a star appears to be. In this system, a difference of 5 magnitudes corresponds to a factor of 100 in brightness. Negative numbers are assigned for objects brighter than magnitude +1.

Basic Properties of Stars Absolute Magnitude Apparent magnitude does not take distance into account. Absolute magnitude is the brightness an object would have if it were placed at a distance of 10 pc.

Basic Properties of Stars Luminosity Luminosity is the energy output from the surface of a star per second. The brightness we observe for a star depends on both its luminosity and its distance.

Spectra of Stars Classification Stars are assigned spectral types in the following order: O, B, A, F, G, K, and M. The classes correspond to stellar temperatures, with the O stars being the hottest and the M stars being the coolest. The Sun is a type G2 star, which corresponds to a surface temperature of about 5800 K.

Spectra of Stars Classification All stars, including the Sun, have nearly identical compositions--about 73% of a star’s mass is hydrogen, about 25% is helium, and the remaining 2 % is composed of all the other elements. The differences in the appearance of their spectra are almost entirely a result of temperature effects.

Spectra of Stars H-R Diagram A Hertzsprung-Russell diagram, or H-R diagram, demonstrates the relationship between mass, luminosity, temperature, and the diameter of stars. An H-R diagram plots the absolute magnitude on the vertical axis and temperature or spectral type on the horizontal axis.

Spectra of Stars H-R Diagram The main sequence, which runs diagonally from the upper-left corner to the lower-right corner of an H-R diagram, represents about 90 percent of stars. Red giants are large, cool, luminous stars plotted at the upper-right corner. White dwarfs are small, dim, hot stars plotted in the lower-left corner.

Brighter (-10) Dim (+15) Warmer (35,000 C) Cooler (3,500 C) -------Temperature-----

Canopus

Sun