Science, Part 84

Question 1: Describe a binary star.

Answer 1: Binary star systems, of which about fifty percent of the stars in the sky are members, consist of two stars that orbit each other. The orbits of and distances between members of a binary system vary. A visual binary is a pair of stars that can be visually observed. Positional measurements of a visual binary reveal the orbital paths of the two stars. Astronomers can identify astrometric binaries through long-term observation of a visible star—if the star appears to wobble, it may be inferred that it is orbiting a companion star that is not visible. An eclipsing binary can be identified through observation of the brightness of a star. Variations in the visual brightness of a star can occur when one star in a binary system passes in front of the other. Sometimes, variations in the spectral lines of a star occur because it is in a binary system. This type of binary is a spectroscopic binary.

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Question 2: Explain the Hertzsprung-Russell diagram.

Answer 2: The Hertzsprung-Russell (H-R) diagram was developed to explore the relationships between the luminosities and spectral qualities of stars. This diagram involves plotting these qualities on a graph, with absolute magnitude (luminosity) on the vertical and spectral class on the horizontal. Plotting a number of stars on the H-R diagram demonstrates that stars fall into narrowly defined regions, which correspond to stages in stellar evolution. Most stars are situated in a diagonal strip that runs from the top-left (high temperature, high luminosity) to the lower-right (low temperature, low luminosity). This diagonal line shows stars in the main sequence of evolution (often called dwarfs). Stars that fall above this line on the diagram (low temperature, high luminosity) are believed to be much larger than the stars on the main sequence (because their high luminosities are not due to higher temperatures than main sequence stars); they are termed giants and supergiants. Stars below the main sequence (high temperature, low luminosity) are called white dwarfs. The H-R diagram is useful in calculating distances to stars.

Question 3: Explain stellar evolution.

Answer 3: The life cycle of a star is closely related to its mass—low-mass stars become white dwarfs, while high-mass stars become supernovae. A star is born when a protostar is formed from a collapsing interstellar cloud. The temperature at the center of the protostar rises, allowing nucleosynthesis to begin. Nucleosynthesis, or hydrogen-burning through fusion, entails a release of energy. Eventually, the star runs out of fuel (hydrogen). If the star is relatively low mass, the disruption of hydrostatic equilibrium allows the star to contract due to gravity. This raises the temperature just outside the core to a point at which nucleosynthesis and a different kind of fusion (with helium as fuel) that produces a carbon nucleus can occur. The star swells with greater energy, becoming a red giant. Once this phase is over, gravity becomes active again, shrinking the star until the degeneracy pressure of electrons begins to operate, creating a white dwarf that will eventually burn out. If the star has a high mass, the depletion of hydrogen creates a supernova.

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