7. Stars and Distances Flashcards
How are stellar masses distributed?
Few stars with masses up to about 100 times that of the Sun. (a few out of several billion) may have masses as large as 250 solar masses.
How can we see through thick clouds to find young stars?
astronomers studying them rely on infrared, radio, and X- ray light to see through the dust.
Approximately what mass star is most common?
We estimate that about 90% of the true stars overall (excluding brown dwarfs) in our part of space are main-sequence stars, about 10% are white dwarfs, and fewer than 1% are giants or supergiants. Main sequence mass range 0.4 to 40 solar masses.
It is almost impossible to determine masses for individual stars.
False. The binary star systems are the key: Once we have calculated the masses for a whole lot of stars in binary systems, and also know howluminousthey are, we notice that there is a relationship between theirluminosityand their mass. In other words, for single stars we only need to measure its luminosity and then use themass-luminosity relationto figure out their mass
Binary stars can be used to determine stellar masses, using different techniques.
Plug in the mean distance between the two stars (example: 19.8 AU) and their orbital period (example: 50.1 Earth-years) into the easy-to-use formula below, first derived by Johannes Kepler in 1618, and known asKepler’s Third law:
Total mass = distance^3 /period^2
Total mass = 19.8^3 /50.1^2
Total mass = 7762.39/2510.01 = 3.09 times the sun’s mass
Explain the difference between a visual binary and an optical double.
Optical doubles are unrelated stars that appear close together through chance alignment with Earth.Visual binaries are gravitationally-bound stars that are separately visible with a telescope.
Explain what is a spectroscopic binary and how they appear
Spectroscopic binary stars are found from observations ofradialvelocity.
Brighter member of a binary can have a continuously changing periodic velocity.
Periodic velocity alters the wavelengths of its spectral lines in a rhythmic way; the velocity curve repeats itself exactly from one cycle to the next, and the motion can be interpreted as orbital motion
Explain what is an eclipsing binary and how they appear
An eclipsing binary consists of two close stars moving in anorbitso placed in space in relation toEarththat thelightof one can at times be hidden behind the other. Depending on the orientation of the orbit and sizes of the stars, the eclipses can be total or annular (in the latter, a ring of one star shows behind the other at the maximum of the eclipse) or both eclipses can be partial.
Explain how a spectroscopic binary can be used to determine the (total) mass of the two stars
If spectra from both members are observed, mass ratios can be found. If one spectrum alone is observed, only a quantity called themass function can be derived, from which is calculated a lower limit to the stellar masses. If a spectroscopic binary is also observed to be aneclipsingsystem, the inclination of the orbit and often the values of the individual masses can beascertained.
Explain how an eclipsing binary can be used to determine the size of the stars
Eclipsing binaries are used to determine indirectly the diameters of stars. These are two stars orbiting each other in a plane that is parallel to your line of sight so you see their orbits edge-on. From the time intervals between contacts, it is possible to estimate the diameters of the two stars.
Describe the mass-luminosity relation
Mass–luminosity relation is an equation giving the relationship between a star’s mass and its luminosity
Explain why distance is so crucial for astronomy.
By knowing the distance to an object we can learn about its true size, the energy it produces, the luminosity. We need to know how far away objects are to look at differences between them
Describe how parallax works and define a parsec.
The parallax formula states that the distance to a star is equal to 1 divided by the parallax angle, p, where p is measured in arc-seconds, and d is parsecs.
d= 1/p
Parsec: a unit of distance used in astronomy, equal to about 3.26 light years (3.086 × 10^13 kilometres). One parsec corresponds to the distance at which the mean radius of the earth’s orbit subtends an angle of one second of arc
Explain how radar is used for distances and explain what objects are suitable for this method.
Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwaves off target objects and analyzing the reflections. The maximum range of astronomy by radar is very limited, and is confined to the Solar System.
Explain why satellites are better for determining parallax distances.
The earth’s atmosphere limits the accuracy of parallax measurements from the ground, by limiting the resolution (sharpness) of a stellar image. Sharper images, and therefore more accurate parallax measurements, require getting above the earth’s atmosphere.
Explain the Cepheid method for determining distances.
Cepheid variables are extremely luminous and very distant ones can be observed and measured. Once the period of a distant Cepheid has been measured, its luminosity can be determined from the known behavior of Cepheid variables. Then its absolute magnitude and apparent magnitude can be related by the distance modulus equation, and its distance can be determined.
Wien’s Law
