Stellar Parallax, Magnitudes, and Hertzsprung-Russell Diagram Insights

Parallax

apparent shift in position of an object as the observer moves

Stellar parallax

apparent shift seen in the position of a nearby star against more distant stars as the Earth orbits the Sun.

Definition of stellar parallax

Stellar parallax is the angle subtended by the radius of Earth's orbit as seen from the star.

Cause of stellar parallax

Stellar parallax appears because Earth moves in space.

Apparent magnitude

a measure of a star's brightness, as seen from Earth.

Comparison of apparent magnitudes +1 and +2

A star of apparent magnitude +1 appears Brighter than a star of apparent magnitude +2.

Comparison of apparent magnitudes -2 and -3

A star of apparent magnitude -2 appears Fainter than a star of apparent magnitude -3.

Difference between absolute and apparent magnitude

Absolute magnitude is an intrinsic property of the star, whereas apparent magnitude depends on its distance from Earth.

Relation of apparent and absolute magnitude

For most stars, the apparent magnitude is a larger number than the absolute magnitude.

Observations needed for absolute magnitude

apparent magnitude and distance

Luminosity of a star

total energy emitted by the star into all space per second, measured in watts

Determining the Sun's surface composition

The chemical makeup of the Sun's surface can be determined by solar spectroscopy.

Spectral type O stars

Stars of spectral type O, the hottest blue-white stars, have spectra characterized by very few spectral lines.

Hertzsprung-Russell diagram

a plot of absolute magnitude (or intrinsic brightness) against temperature of a group of stars.

Physical parameters in Hertzsprung-Russell diagram

luminosity and surface temperature

Stars on the H-R diagram

As you move upward and to the left on the H-R diagram the stars become hotter and whiter.

Comparison of stars in H-R diagram

Compared with a star in the middle of the Hertzsprung-Russell diagram, a star in the lower left of the diagram is Smaller.

Location of most local stars

Where on the Hertzsprung-Russell diagram do most local stars in our universe congregate? main sequence, where stars are generating energy by fusion reactions.

Fraction of main-sequence stars

What fraction of the stars surrounding the Sun are main-sequence stars? almost all, about 90%.

Dwarf star

main-sequence star

White dwarf star

star of about the same size (diameter) as Earth

Faintness of white dwarfs

White dwarfs are very small.

High intrinsic brightness star

Measurements indicate that a certain star has a very high intrinsic brightness (100,000 times as bright as the Sun) and yet is relatively cool (3500 K). The star must be very large.

Radii of stars with same luminosity

The hotter star has a quarter the radius of the cooler star.

Luminosity class determination

By what standard technique do astronomers find the luminosity class (I, II, III, IV, or V) of a star? studying the absorption lines in the star's spectrum.

White dwarfs and luminosity classification

White dwarfs are not included in the luminosity classification because they are no longer producing energy by nuclear reactions.

Center of mass in binary stars

When two stars of unequal mass orbit each other under their mutual gravitational attraction, the center of mass of the system is located at the point between the two stars, closer to the more massive star.

Measuring mass of a star

What is the only way to measure the mass of a star accurately? Measure its gravitational effect on another object.

Importance of binary star systems

One important aspect of the study of binary star systems, as distinct from single stars, is that it provides a measurement of the masses of stars.