The Astrophysics Spectator



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The Structure of Our Universe


The study of the stars is largely a modern preoccupation. The ancient astronomers mapped the positions and brightness of the stars, but, without telescopes and without a strong understanding of the physics, further progress was impossible. No progress was possible before the invention of the telescope, because the distances to stars cannot be derived without the aid of a telescope. Total understanding of stars was not achieved until the 20th century, when astrophysicists developed the physics of thermonuclear fusion.

Each star at some point in its existence generates energy through thermonuclear fusion. A star at birth is a ball of gas composed by mass of about 75% hydrogen and 25% helium. In the first stage of its thermonuclear life, a star converts hydrogen into helium. This fusion stage distinguishes a star from a brown dwarf or planet; only objects with a mass greater than 7.5% of the Sun's mass can drive hydrogen fusion, and so only these objects are stars. The hydrogen burning stage is the longest thermonuclear stage for a star; it can last as short as several million years for the most massive stars, or as long as several hundred billion years for the least massive stars. The small, long-lived stars are red and dim, and the large, short-lived stars are brilliant and blue. Most stars in our Galaxy are in this hydrogen-burning stage.

Our own sun is a star in this hydrogen-burning phase. This stage is expected to last 9 billion years; already the Sun has burned hydrogen for half of this time. The moderate mass means that the Sun's surface temperature is moderate in value, 4,800° Kelvin, which causes the Sun to radiate predominately yellow light.

The later stages of a star's thermonuclear life convert helium to carbon, oxygen, and other heavier elements. Stars in these stages are brilliant but red. A star's helium fusion stage is always much shorter than its hydrogen fusion stage, and the stages were heavier elements are burned are even shorter.

The eventual end of fusion within a large star can lead to the star's explosion in an event called a supernova. The remnant of this explosion can be either a small, compact star—a neutron star—or a black hole. Smaller stars like the Sun collapse to a compact star called a degenerate dwarf—also known as a white dwarf—after thermonuclear fusion ceases. These collapsed stars can be brilliant after they form, but over time they cool and grow dim, eventually becoming invisible.

The largest stars, which are very uncommon, are several hundred times the mass of the Sun. Stars larger than this are not observed; they apparently cannot form. On the other hand, smallest stars, the stars that are just above the hydrogen fusion threshold of 0.075 solar masses, are extremely common. But which of these two types of star do we see? While the large stars are uncommon, they generate tremendous power, so that they are visible at great distances, and the small stars, while numerous, produce so little power that they are only visible when nearby. So which property is dominant, the visibility of massive stars at great distance, or the large numbers of nearby low-mass stars?

When we list the brightest stars in the sky, we find that most of them are much larger than the Sun, emitting many times as much power as the Sun. The brightest star in the sky, Sirius (&alpha Canis Majoris), is only 2.7 parsecs away; it radiates 23 times the power radiated by the Sun, although it has only 2.6 times the Sun's mass. Among the brightest stars in our sky is Rigel (β Orionis), in the Orion constellation. At 77 parsecs away, this star is among the most distant of the bright stars; it compensates by radiating 145,000 times as much power as the Sun, despite containing only 37 times the Sun's mass. All twenty-five of the brightest stars generate more power than the Sun, and 15 of these stars generate more than 100 times the Sun's power. The closest of the 15 brightest stars is almost 19 parsecs away.

With the exception of Sirius and Procyon (&alpha Canis Minoris), the brightest stars in the sky are not the nearest stars. The nearest star, Proxima (V645 Centauri), in the Centaurus constellation, is 1.3 parsecs away. It is not visible without a telescope, as it radiates only 1/19,000 times the power of the Sun. The next closest star, Rigil Kentaurus (α Centauri), is slightly brighter than the Sun, and is the third-brightest star in the sky. But of the the 11 known stars within 3 parsecs of the Sun, 9 are less luminous than the Sun, and 8 are invisible to our eyes. Most of these nearby stars resemble Proxima in their power generation.

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