Astronomy

A

8

Black Holes

Black holes are objects that are so massive and dense that their immense gravitational pull does not even let light escape. If the core left over after a supernova explosion has a mass of more than about five times that of the Sun, the force holding up the neutrons in the core is not large enough to balance the inward gravitational force. No outward force is large enough to resist the gravitational force. The core of the star continues to collapse. When the core's mass is sufficiently concentrated, the gravitational force of the core is so strong that nothing, not even light, can escape it. The gravitational force is so strong that classical physics no longer applies, and astronomers use Einstein’s general theory of relativity to explain the behavior of light and matter under such strong gravitational forces. According to general relativity, space around the core becomes so warped that nothing can escape, creating a black hole. A star with a mass ten times the mass of the Sun would become a black hole if it were compressed to 90 km (60 mi) or less in diameter.

Astronomers have various ways of detecting black holes. When a black hole is in a binary system, matter from the companion star spirals into the black hole, forming a disk of gas around it. The disk becomes so hot that it gives off X rays that astronomers can detect from Earth. Astronomers use X-ray telescopes in space to find X-ray sources, and then they look for signs that an unseen object of more than about five times the mass of the Sun is causing gravitational tugs on a visible object.

B

Star Locations

The basic method that astronomers use to find the distance of a star from Earth uses parallax. Parallax is the change in apparent position of a distant object when viewed from different places. For example, imagine a tree standing in the center of a field, with a row of buildings at the edge of the field behind the tree. If two observers stand at the two front corners of the field, the tree will appear in front of a different building for each observer. Similarly, a nearby star's position appears slightly different when seen from different angles.

Parallax also allows human eyes to judge distance. Each eye sees an object from a slightly different angle. The brain compares the two pictures to judge the distance to the object. Astronomers use the same idea to calculate the distance to a star. Stars are very far away, so astronomers must look at a star from two locations as far apart as possible to get a measurement. The movement of Earth around the Sun makes this possible. By taking measurements six months apart from the same place on Earth, astronomers take measurements from locations separated by the diameter of Earth’s orbit. That is a separation of about 300 million km (186 million mi). The nearest stars will appear to shift slightly with respect to the background of more distant stars. Even so, the greatest stellar parallax is only about 0.77 seconds of arc, an amount 4,600 times smaller than a single degree. Astronomers calculate a star’s distance by dividing 1 by the parallax. Distances of stars are usually measured in parsecs. A parsec is 3.26 light-years, and a light-year is the distance that light travels in a year, or about 9.5 trillion km (5.9 trillion mi). Proxima Centauri, the Sun’s nearest neighbor, has a parallax of 0.77 seconds of arc. This measurement indicates that Proxima Centauri’s distance from Earth is about 1.3 parsecs, or 4.2 light-years. Because Proxima Centauri is the Sun’s nearest neighbor, it has a larger parallax than any other star.

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Astronomers can measure stellar parallaxes for stars up to about 500 light-years away, which is only about 2 percent of the distance to the center of our galaxy. Beyond that distance, the parallax angle is too small to measure.

A European Space Agency spacecraft named Hipparcos (an acronym for High Precision Parallax Collecting Satellite), launched in 1989, gave a set of accurate parallaxes across the sky that was released in 1997. This set of measurements has provided a uniform database of stellar distances for over 100,000 stars and a somewhat less accurate database of over 1 million stars. These parallax measurements provide the base for measurements of the distance scale of the universe. Hipparcos data are leading to more accurate age calculations for the universe and for objects in it, especially globular clusters of stars.

C

Starlight

Astronomers use a star’s light to determine the star’s temperature, composition, and motion. Astronomers analyze a star’s light by looking at its intensity at different wavelengths. Blue light has the shortest visible wavelengths, at about 400 nanometers. (A nanometer, abbreviated nm, is one billionth of a meter, or about one forty-thousandth of an inch.) Red light has the longest visible wavelengths, at about 650 nm. A law of radiation known as Wien's displacement law (developed by German physicist Wilhelm Wien) links the wavelength at which the most energy is given out by an object and its temperature. A star like the Sun, whose surface temperature is about 6000 K (about 5730°C or about 10,350°F), gives off the most radiation in yellow-green wavelengths, with decreasing amounts in shorter and longer wavelengths. Astronomers put filters of different standard colors on telescopes to allow only light of a particular color from a star to pass. In this way, astronomers determine the brightness of a star at particular wavelengths. From this information, astronomers can use Wien’s law to determine the star’s surface temperature.

Astronomers can see the different wavelengths of light of a star in more detail by looking at its spectrum. The continuous rainbow of color of a star's spectrum is crossed by dark lines, or spectral lines. In the early 19th century, German physicist Josef Fraunhofer identified such lines in the Sun's spectrum, and they are still known as Fraunhofer lines. American astronomer Annie Jump Cannon divided stars into several categories by the appearance of their spectra. She labeled them with capital letters according to how dark their hydrogen spectral lines were. Later astronomers reordered these categories according to decreasing temperature. The categories are O, B, A, F, G, K, and M, where O stars are the hottest and M stars are the coolest. The Sun is a G star. An additional spectral type, L stars, was suggested in 1998 to accommodate some cool stars studied using new infrared observational capabilities. Detailed study of spectral lines shows the physical conditions in the atmospheres of stars. Careful study of spectral lines shows that some stars have broader lines than others of the same spectral type. The broad lines indicate that the outer layers of these stars are more diffuse, meaning that these layers are larger, but spread more thinly, than the outer layers of other stars. Stars with large diffuse atmospheres are called giants. Giant stars are not necessarily more massive than other stars—the outer layers of giant stars are just more spread out.

Many stars have thousands of spectral lines from iron and other elements near iron in the periodic table. Other stars of the same temperature have relatively few spectral lines from such elements. Astronomers interpret these findings to mean that two different populations of stars exist. Some formed long ago, before supernovas produced the heavy elements, and others formed more recently and incorporated some heavy elements. The Sun is one of the more recent stars.

Spectral lines can also be studied to see if they change in wavelength or are different in wavelength from sources of the same lines on Earth. These studies tell us, according to the Doppler effect, how much the star is moving toward or away from us. Such studies of starlight can tell us about the orbits of stars in binary systems or about the pulsations of variable stars, for example.

VII

Galaxies

Astronomers study galaxies to learn about the structure of the universe. Galaxies are huge collections of billions of stars. Our Sun is part of the Milky Way Galaxy. Galaxies also contain dark strips of dust and may contain huge black holes at their centers. Galaxies exist in different shapes and sizes. Some galaxies are spirals, some are oval, or elliptical, and some are irregular. The Milky Way is a spiral galaxy. Galaxies tend to group together in clusters.

A

The Milky Way

Our Sun is only one of about 400 billion stars in our home galaxy, the Milky Way. On a dark night, far from outdoor lighting, a faint, hazy, whitish band spans the sky. This band is the Milky Way Galaxy as it appears from Earth. The Milky Way looks splotchy, with darker regions interspersed with lighter ones.

The Milky Way Galaxy is a pinwheel-shaped flattened disk about 75,000 light-years in diameter. The Sun is located on a spiral arm about two-thirds of the way out from the center. The galaxy spins, but the center spins faster than the arms. At Earth’s position, the galaxy makes a complete rotation about every 200 million years.

When observers on Earth look toward the brightest part of the Milky Way, which is in the constellation Sagittarius, they look through the galaxy’s disk toward its center. This disk is composed of the stars, gas, and dust between Earth and the galactic center. When observers look in the sky in other directions, they do not see as much of the galaxy’s gas and dust, and so can see objects beyond the galaxy more clearly.

The Milky Way Galaxy has a core surrounded by its spiral arms. A spherical cloud containing about 100 examples of a type of star cluster known as a globular cluster surrounds the galaxy. Still farther out is a galactic corona. Astronomers are not sure what types of particles or objects occupy the corona, but these objects do exert a measurable gravitational force on the rest of the galaxy.

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