The Milky Way and Beyond by Britannica Educational Publishing
Authors: Britannica Educational Publishing
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Cosmic Background Explorer first determined a reliable value for the velocity and direction of solar motion with respect to the nearby universe. The solar system is headed toward the constellation Leo with a velocity of 370 km/sec (230 miles/sec). This value was confirmed in the 2000s by an even more sensitive space telescope, the Wilkinson Microwave Anisotropy Probe.
CHAPTER 2
S TARS
T he Milky Way Galaxy is made up of one hundred billion of those tantalizing points of light called stars, the massive, self-luminous celestial bodies of gas that shine by radiation derived from their internal energy sources. Our Sun is a star. Of the tens of billions of trillions of stars composing the observable universe, only a very small percentage are visible to the naked eye. Many stars occur in pairs, multiple systems, and star clusters. Members of such stellar groups are physically related through common origin and bound by mutual gravitational attraction. Somewhat related to star clusters are stellar associations, which consist of loose groups of physically similar stars insufficient mass as a group to remain together as an organization.
THE NATURE OF STARS
----
To say that stars are balls of gas that shine through the workings of their internal energy does not do justice to their full nature and complexity. Not all stars are like our Sun. Some stars are massive giants doomed to burn away in merely millions of years. Others are dim brown dwarfs that are in some ways like stars and in others like the even smaller giant planets.
S IZE AND A CTIVITY
The Sun seems like an impressive star. It casts aside the gloom of night and bathes the entire planet in its life-givingrays. However, when the Sun is considered among stars, it is merely average in its size and the activity of its winds.
V ARIATIONS IN S TELLAR S IZE
With regard to mass, size, and intrinsic brightness, the Sun is a typical star. Its approximate mass is 2 Ã 10 30 kg (about 330,000 Earth masses), its approximate radius 700,000 km (430,000 miles), and its approximate luminosity 4 Ã 10 33 ergs per second (or equivalently 4 Ã 10 23 kilowatts of power). Other stars often have their respective quantities measured in terms of those of the Sun.
The table lists data pertaining to the 20 brightest stars, or, more precisely, stellar systems, since some of them are double (binary stars) or even triple stars. Successive columns give the name of the star, its brightness expressed in units called visual magnitudes and the spectral type or types to which the star or its components belong, the distance in light-years (a light-year being the distance that light waves travel in one Earth year: 9.46 trillion km, or 5.88 trillion miles), and the visual luminosity in terms of that of the Sun. All the primary stars (designated as the A component) are intrinsically as bright as or brighter than the Sun. Some of the companion stars are fainter.
Many stars vary in the amount of light they radiate. Stars such as Altair, Alpha Centauri A and B, and Procyon A are called dwarf stars. Their dimensions are roughly comparable to those of the Sun. Sirius A and Vega, though much brighter, also are dwarf stars; their higher temperatures yield a larger rate of emission per unit area. Aldebaran A, Arcturus, and Capella A are examples of giant stars, whose dimensions are much larger than those of the Sun. Observations with an interferometer (an instrument that measures the angle subtended by the diameter of a star at the observerâs position), combined with parallax measurements, which yield a starâs distance, give sizes of 12 and 22 solar radii for Arcturus and Aldebaran A. Betelgeuse and Antares A are examples of supergiant stars. The latter has a radius some 300 times that of the Sun, whereas the variable star Betelgeuse oscillates between roughly 300 and 600 solar radii.
Several of the stellar class of white dwarf stars, which have low luminosities and high densities, also are listed.
CHAPTER 2
S TARS
T he Milky Way Galaxy is made up of one hundred billion of those tantalizing points of light called stars, the massive, self-luminous celestial bodies of gas that shine by radiation derived from their internal energy sources. Our Sun is a star. Of the tens of billions of trillions of stars composing the observable universe, only a very small percentage are visible to the naked eye. Many stars occur in pairs, multiple systems, and star clusters. Members of such stellar groups are physically related through common origin and bound by mutual gravitational attraction. Somewhat related to star clusters are stellar associations, which consist of loose groups of physically similar stars insufficient mass as a group to remain together as an organization.
THE NATURE OF STARS
----
To say that stars are balls of gas that shine through the workings of their internal energy does not do justice to their full nature and complexity. Not all stars are like our Sun. Some stars are massive giants doomed to burn away in merely millions of years. Others are dim brown dwarfs that are in some ways like stars and in others like the even smaller giant planets.
S IZE AND A CTIVITY
The Sun seems like an impressive star. It casts aside the gloom of night and bathes the entire planet in its life-givingrays. However, when the Sun is considered among stars, it is merely average in its size and the activity of its winds.
V ARIATIONS IN S TELLAR S IZE
With regard to mass, size, and intrinsic brightness, the Sun is a typical star. Its approximate mass is 2 Ã 10 30 kg (about 330,000 Earth masses), its approximate radius 700,000 km (430,000 miles), and its approximate luminosity 4 Ã 10 33 ergs per second (or equivalently 4 Ã 10 23 kilowatts of power). Other stars often have their respective quantities measured in terms of those of the Sun.
The table lists data pertaining to the 20 brightest stars, or, more precisely, stellar systems, since some of them are double (binary stars) or even triple stars. Successive columns give the name of the star, its brightness expressed in units called visual magnitudes and the spectral type or types to which the star or its components belong, the distance in light-years (a light-year being the distance that light waves travel in one Earth year: 9.46 trillion km, or 5.88 trillion miles), and the visual luminosity in terms of that of the Sun. All the primary stars (designated as the A component) are intrinsically as bright as or brighter than the Sun. Some of the companion stars are fainter.
Many stars vary in the amount of light they radiate. Stars such as Altair, Alpha Centauri A and B, and Procyon A are called dwarf stars. Their dimensions are roughly comparable to those of the Sun. Sirius A and Vega, though much brighter, also are dwarf stars; their higher temperatures yield a larger rate of emission per unit area. Aldebaran A, Arcturus, and Capella A are examples of giant stars, whose dimensions are much larger than those of the Sun. Observations with an interferometer (an instrument that measures the angle subtended by the diameter of a star at the observerâs position), combined with parallax measurements, which yield a starâs distance, give sizes of 12 and 22 solar radii for Arcturus and Aldebaran A. Betelgeuse and Antares A are examples of supergiant stars. The latter has a radius some 300 times that of the Sun, whereas the variable star Betelgeuse oscillates between roughly 300 and 600 solar radii.
Several of the stellar class of white dwarf stars, which have low luminosities and high densities, also are listed.
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