Universe and Solar System
2.2 Galaxies and Stars
Types of Galaxies
Galaxies are the most prominent groups of stars and can contain anywhere from a few million stars to many billions of stars. Every star that is visible in the night sky is part of the Milky Way Galaxy. To the naked eye, the closest major galaxy, the Andromeda Galaxy, looks like only a dim, fuzzy spot, but that fuzzy spot contains one trillion stars.
Spiral and Elliptical Galaxies
Spiral galaxies spin, so they appear as a rotating disk of stars and dust, with a bulge in the middle, like the Sombrero Galaxy. Several arms spiral outward in the Pinwheel Galaxy and are appropriately called spiral arms. Spiral galaxies have lots of gas and dust and lots of young stars.
Other galaxies are egg-shaped and called an elliptical galaxy. The smallest elliptical galaxies are as small as some globular clusters. Giant elliptical galaxies, on the other hand, can contain over a trillion stars. Elliptical galaxies are reddish to yellowish because they contain mostly old stars. Most elliptical galaxies contain very little gas and dust because they had already formed. However, some elliptical galaxies contain lots of dust. Why might some elliptical galaxies contain dust?
Irregular and Dwarf Galaxies
Galaxies that are not elliptical galaxies or spiral galaxies are irregular galaxies. Most irregular galaxies were once spiral or elliptical galaxies that were then deformed either by gravitational attraction to a more massive galaxy or by a collision with another galaxy.
Dwarf galaxies are small galaxies containing only a few million to a few billion stars. Dwarf galaxies are the most common type in the universe. However, because they are relatively small and dim, we do not see as many dwarf galaxies from Earth. Most dwarf galaxies are irregular in shape. However, there are also dwarf elliptical galaxies and dwarf spiral galaxies.
Look back at the picture of the spiral galaxy, Andromeda. Next to our closest galaxy neighbor are two dwarf elliptical galaxies that are companions to the Andromeda Galaxy. One is a bright sphere to the left of center, and the other is a long ellipse below and to the center’s right. Dwarf galaxies are often found near more massive galaxies. They sometimes collide with and merge into their larger neighbors.
Milky Way Galaxy
On a dark, clear night, a milky band of light will stretch across the sky. This band is the disk of a galaxy, the Milky Way Galaxy is our galaxy and is made up of millions of stars along with a lot of gas and dust. Although it is difficult to know what the shape of the Milky Way Galaxy is because we are inside of it, astronomers have identified it as a typical spiral galaxy containing about 100 billion to 400 billion stars.
Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. The disk is about 100,000 light-years across and 3,000 light-years thick. Most of the Galaxy’s gas, dust, young stars, and open clusters are in the disk. Scientists know that the Milky Way is a spiral galaxy because of the shape of the galaxy as from Earth’s perspective, the velocities of stars and gas in the galaxy show a rotational motion, and the gases, color, and dust are typical of spiral galaxies.
The central bulge is about 12,000 to 16,000 light-years wide and 6,000 to 10,000 light-years thick. The central bulge contains mostly older stars and globular clusters. Some recent evidence suggests the bulge might not be spherical, but is instead shaped like a bar. The bar might be as long as 27,000 light-years long. The disk and bulge are surrounded by a faint, spherical halo, which also includes old stars and globular clusters. Astronomers have discovered that there is a gigantic black hole at the center of the galaxy.
The Milky Way Galaxy is a significant place. Our solar system, including the Sun, Earth, and all the other planets, is within one of the spiral arms in the disk of the Milky Way Galaxy. Most of the stars we see in the sky are relatively nearby stars that are also in this spiral arm. Earth is about 26,000 light-years from the center of the galaxy, a little more than halfway out from the center of the galaxy to the edge.
Just as Earth orbits the Sun, the Sun and solar system orbit the center of the Milky Way Galaxy. One orbit of the solar system takes about 225 to 250 million years. The solar system has orbited 20 to 25 times since it formed 4.6 billion years ago. Astronomers have recently found that at the center of the Milky Way, and most other galaxies, is a supermassive black hole, though a black hole cannot be seen.
Nuclear Fusion within Stars
The Sun is Earth’s primary source of energy, yet the planet only receives a small portion of its energy, and the Sun is just an ordinary star. Many stars produce much more energy than the Sun. The energy source for all stars is nuclear fusion.
Stars are made mostly of hydrogen and helium, which are packed so densely in a star that is the star’s center. The pressure is enormous enough to initiate nuclear fusion reactions. In a nuclear fusion reaction, the nuclei of two atoms combine to create a new atom. Most commonly, two hydrogen atoms fuse to become a helium atom in the core of a star. Although nuclear fusion reactions require much energy to get started, they produce enormous amounts of energy once they are going.
In a star, the energy from fusion reactions in the core pushes outward to balance the inward pull of the star’s gravity. This energy moves outward through the star’s layers until it finally reaches the star’s outer surface. The star’s outer layer glows brightly, sending the energy into space as electromagnetic radiation, including visible light, heat, ultraviolet light, and radio waves.
In particle accelerators, subatomic particles are propelled until they have attained almost the same amount of energy as found in the core of a star. When these particles collide head-on, new particles are created. This process simulates the nuclear fusion that takes place in the cores of stars. The process also mimics the conditions that allowed for the first helium atom to be produced from the collision of two hydrogen atoms in the first few minutes of the universe.
Think about how the color of a piece of metal changes with temperature. A coil of an electric stove will start black, but with added heat will start to glow a dull red. With more heat, the coil turns a brighter red, then orange. At extremely high temperatures, the coil will turn yellow-white or even blue-white. A star’s color is also determined by the temperature of the star’s surface. Relatively cool stars are red, warmer stars are orange or yellow, and extremely hot stars are blue or blue-white.
Color is the most common way to classify stars. A star’s class is given by a letter, where each letter corresponds to a color and temperature range. Note that these letters do not match the color names; they are leftover from an older system that is no longer used. For most stars, the surface temperature is also related to size. Bigger, bluish-white stars produce more energy and have hotter surfaces than smaller, yellow stars that produce less energy, so their surfaces are hotter than smaller stars. These stars tend toward bluish-white.
Stars have a life cycle that is expressed similarly to the life cycle of a living creature: they are born, grow, change over time, and eventually die. Most stars vary in size, color, and class at least once in their lifetime. What astronomers know about the life cycles of stars is because of data gathered from visual, radio, and X-ray telescopes.
Main Sequence Stars
For most of a star’s life, nuclear fusion in the core produces helium from hydrogen, a stage called a main-sequence star. This term comes from the Hertzsprung-Russell diagram shown below. For stars on the main sequence, the temperature is directly related to brightness. A star is on the main sequence as long as it can balance the inward force of gravity with the outward force of nuclear fusion in its core. The more massive a star, the more it must burn hydrogen fuel to prevent internal gravitational collapse. Because they burn more fuel, massive stars have higher temperatures, but run out of hydrogen sooner than smaller stars.
Our Sun, a yellow star, has been a main-sequence star for about 5 billion years and will continue on the main sequence for about 5 billion more years. Massive stars may be on the main sequence for only 10 million years. Tiny stars may last tens to hundreds of billions of years.
Red Giants and White Dwarfs
As a star begins to use up its hydrogen, it fuses helium atoms into heavier atoms such as carbon. A blue giant star has exhausted its hydrogen fuel and is a transitional phase. When the light elements are mostly used up, the star can no longer resist gravity, and it starts to collapse inward. The outer layers of the star grow outward and cool. The larger, cooler star turns red, and so is called a red giant.
Eventually, a red giant burns up all of the helium in its core. What happens next depends on how massive the star is. A typical star, such as the Sun, stops fusion completely. Gravitational collapse shrinks the star’s core to a white, glowing object about Earth’s size, called a white dwarf, which will ultimately fade out.
Low Mass Stars
High Mass Stars
A star that runs out of helium will end its life much more dramatically. When very massive stars leave the main sequence, they become red supergiants. Unlike a red giant, when all the helium in a red supergiant is gone, fusion continues. Lighter atoms fuse into heavier atoms up to iron atoms. Creating elements heavier than iron through fusion uses more energy than it produces, so stars do not ordinarily form any heavier elements. When there are no more elements for the star to fuse, the core succumbs to gravity and collapses, creating an explosion called a supernova.
A supernova explosion contains so much energy that atoms can fuse to produce heavier elements such as gold, silver, and uranium. A supernova can shine as brightly as an entire galaxy for a short time. Nuclear fusion in stars created all elements with an atomic number more significant than that of lithium.
Neutron Stars and Black Holes
After a supernova explosion, the leftover material in the core is incredibly dense. If the core is less than about four times the mass of the Sun, the star becomes a neutron star. A neutron star is made almost entirely of neutrons, relatively large particles that have no electrical charge.
If the core remaining after a supernova is more than about five times the mass of the Sun, the core collapses into a black hole. Black holes are so dense that not even light can escape their gravity. With no light, a black hole cannot be observed directly. However, a black hole can be identified by the effect that it has on objects around it, and by radiation that leaks out around its edges.
Observing and Measuring Stars
Parallax is an apparent shift in position that takes place when the position of the observer changes. To see an example of parallax, try holding your finger about 30 cm (1 foot) in front of your eyes. Now, while focusing on your finger, close one eye and then the other. Alternate back and forth between eyes, and pay attention to how your finger appears to move. The shift in the position of your finger is an example of parallax. Now try moving your finger closer to your eyes, and repeat the experiment and notice any differences. The closer your finger is to your eyes, the more significant the position changes because of parallax.
Astronomers use this same principle to measure the distance to stars. Instead of a finger, they focus on a star, and instead of switching back and forth between eyes, they switch between the most significant possible differences in observing position. To do this, an astronomer first looks at the star from one position and notes where the star is relative to more distant stars. Now, where will the astronomer go to observe the most significant distance from the first observation? In six months, after Earth moves from one side of its orbit around the Sun to the other side, the astronomer looks at the star again. This time parallax causes the star to appear in a different position relative to more distant stars. From the size of this shift, astronomers can calculate the distance to the star.
Using NASA’s Hubble Space Telescope, astronomers now can precisely measure the distance of stars up to 10,000 light-years away, ten times farther than previously possible. Astronomers have developed yet another novel way to use the 24-year-old space telescope by employing a technique called spatial scanning, which dramatically improves Hubble’s accuracy for making angular measurements. The technique, when applied to the age-old method for gauging distances called astronomical parallax, extends Hubble’s tape measure ten times farther into space. “This new capability is expected to yield new insight into the nature of dark energy, a mysterious component of space that is pushing the universe apart at an ever-faster rate,” said Noble laureate Adam Riess of the Space Telescope Science Institute (STScI) in Baltimore, Md.
Parallax, a trigonometric technique, is the most reliable method for making astronomical distance measurements, and a practice long employed by land surveyors here on Earth. Earth’s orbit’s diameter is the base of a triangle, and the star is the apex where the triangle’s sides meet. The lengths of the sides are calculated by accurately measuring the three angles of the resulting triangle. Astronomical parallax works reliably well for stars within a few hundred light-years of Earth. (Northon, 2014)
For example, measurements of the distance to Alpha Centauri, the star system closest to our sun, vary only by one arc second. This variance in distance is equal to the apparent width of a dime seen from two miles away. Stars farther out have much smaller angles of apparent back-and-forth motion that are extremely difficult to measure. (Northon, 2014)
Astronomers have pushed to extend the parallax yardstick ever deeper into our galaxy by measuring smaller angles more accurately. This new long-range precision was proven when scientists successfully used Hubble to measure the distance of a particular class of bright stars called Cepheid variables, approximately 7,500 light-years away in the northern constellation Auriga. The technique worked so well; they are now using Hubble to measure the distances of other far-flung Cepheids. Such measurements will be used to provide a firmer footing for the so-called cosmic “distance ladder.” This ladder’s “bottom rung” is built on measurements to Cepheid variable stars that, because of their known brightness, have been used for more than a century to gauge the size of the observable universe. They are the first step in calibrating far more distant extra-galactic milepost markers such as Type Ia supernovae.
Riess and the Johns Hopkins University in Baltimore, Md., in collaboration with Stefano Casertano of STScI, developed a technique to use Hubble to make measurements as small as five-billionths of a degree. To make a distance measurement, two exposures of the target Cepheid star were taken six months apart, when Earth was on opposite sides of the sun. A very subtle shift in the star’s position was measured to an accuracy of 1/1,000 the width of a single image pixel in Hubble’s Wide Field Camera 3, which has 16.8 megapixels total. A third exposure was taken after another six months to allow for the team to subtract the effects of the subtle space motion of stars, with additional exposures used to remove other sources of error.
Riess shares the 2011 Nobel Peace Prize in Physics with another team for his leadership in the 1998 discovery the universe’s expansion rate is accelerating — a phenomenon widely attributed to a mysterious, unexplained dark energy filling the universe. This new high-precision distance measurement technique is enabling Riess to gauge just how much the universe is stretching. His goal is to refine estimates of the universe’s expansion rate to the point where dark energy can be better characterized. (Northon, 2014)