Science, Tech, Math › Science Astronomy 101 - Learning About Stars Lesson 5: The Universe Has Gas Share Flipboard Email Print The star cluster Trumpler 14, a collection of stars in the Southern Hemisphere sky. ESO Science Astronomy An Introduction to Astronomy Important Astronomers Solar System Stars, Planets, and Galaxies Space Exploration Chemistry Biology Physics Geology Weather & Climate By Nick Greene Astronomy Expert Nick Greene is a software engineer for the U.S. Navy Space and Naval Warfare Engineering Center. He is also the U.N. World Space Week Coordinator for Antarctica. our editorial process Nick Greene Updated July 03, 2019 Astronomers are often asked about objects in the cosmos and how they came to be. Stars, in particular, fascinate many people, particularly because we can look out on a dark night and see so many of them. So, what are they? Stars are massive shining spheres of hot gas. Those stars you see with your naked eye in the night sky all belong to the Milky Way Galaxy, the huge system of stars that contains our solar system. There are around 5,000 stars which can be seen with the naked eye, though not all stars are visible at all times and places. With a small telescope, hundreds of thousands of stars can be seen. Larger telescopes can show millions of galaxies, which can have upwards of a trillion or more stars. There are more than 1 x 1022 stars in the universe (10,000,000,000,000,000,000,000). Many are so large that if they took our Sun's place, they would engulf Earth, Mars, Jupiter, and Saturn. Others, called white dwarf stars, are around the size of Earth, and neutron stars are less than about 16 kilometers (10 miles) in diameter. Our Sun is about 93 million miles from Earth, 1 astronomical Unit (AU). The difference in its appearance from the stars visible in the night sky is due to its close proximity. The next closest star is Proxima Centauri, 4.2 light-years (40.1 trillion kilometers (20 trillion miles) from Earth. Stars come in a wide variety of colors, ranging from deep red, through orange and yellow to an intense white-blue. The color of a star depends on its temperature. Cooler stars tend to be red, while the hottest ones are blue. Stars are classified many ways, including by their brightness. They are also divided into brightness groups, which are called magnitudes. Each star magnitude is 2.5 times brighter than the next lower star. The brightest stars now represented by negative numbers and they can be dimmer than 31st magnitude. Stars - Stars - Stars Stars are primarily made of hydrogen, smaller amounts of helium, and trace amounts of other elements. Even the most abundant of the other elements present in stars (oxygen, carbon, neon, and nitrogen) are only present in very small quantities. Despite the frequent use of phrases like "the emptiness of space," space is actually full of gases and dust. This material gets compressed by collisions and blast waves from exploding stars, causing lumps of matter to form. If the gravity of these protostellar objects is strong enough, they can pull in other matter for fuels. As they continue to compress, their internal temperatures rise to the point where hydrogen ignites in thermonuclear fusion. While the gravity continues pulling, trying to collapse the star into the smallest possible size, the fusion stabilizes it, preventing further contraction. Thus, a great struggle ensues for the life of the star, as each force continues to push or pull. How Do Stars Produce Light, Heat, and Energy? There are a number of different processes (thermonuclear fusion) which make stars produce light, heat and energy. The most common happens when four hydrogen atoms combine into a helium atom. This releases energy, which is converted to light and heat. Eventually, most of the fuel, hydrogen, is exhausted. As the fuel begins to run out, the strength of the thermonuclear fusion reaction declines. Soon (relatively speaking), gravity will win and the star will collapse under its own weight. At that time, it becomes what is known as a white dwarf. As the fuel further depletes and reaction stops all together, it will collapse further, into a black dwarf. This process can take billions and billions of years to complete. Toward the end of the twentieth century, astronomers began to discover planets orbiting other stars. Because planets are so much smaller and fainter than stars, they are difficult to detect and impossible to see, so how do scientists find them? They measure tiny wobbles in a star’s motion caused by the gravitational pull of the planets. Although no Earth-like planets have been discovered yet, scientists are hopeful. Next lesson, we'll take a closer look at some of these balls of gas.