What is a Star?

Mayra Jaimes, Katie Chamberlain, Christianlly Cena
  • Author
    Mayra Jaimes

    Mayra Jaimes has taught 1st through 10th grade science for over 8 years. She has a bachelor's degree in Biology from the Pontifical Xavierian University in Bogota-Colombia.

  • Instructor
    Katie Chamberlain

    Katie has a PhD in Microbiology and has experience preparing online education content in Biology and Earth Science.

  • Expert Contributor
    Christianlly Cena

    Christianlly has taught college physics and facilitated laboratory courses. He has a master's degree in Physics and is pursuing his doctorate study.

Discover incredible facts about the study of stars. Learn about branches of astronomy, stars' composition, and their life cycle. Updated: 10/16/2021

Table of Contents


The Study of Stars

Astronomy is the scientific study of all celestial bodies, including stars. This science is one of the oldest, probably because humans have always been curious about objects in space. The Moon, the stars, and the planets have been studied for centuries leading scientists to the discovery of motion patterns and other phenomena like the life cycle of stars, solar and lunar eclipses, and many other cosmic events.

A person that studies objects in space and their interactions is called an astronomer. Early astronomers like the Babylonians started to examine the positions of planets and stars in the sky during different times of the year and at different locations of the globe. Many ancient Greek astronomers like Thales of Miletus and Ptolemy made great advances in the understanding of space phenomena. Between the XVI and XVII centuries, prestigious astronomers like the polish Nicolaus Copernicus with his heliocentric theory, and the Italian Galileo Galilei with his discovery of the moons of Jupiter and Venus revolutionized the world using limited technology. On the other hand, modern astronomers use different types of sophisticated equipment like giant telescopes and spectroscopes to make detailed observations of different celestial bodies. They are also able to measure the dimensions of these bodies, the distances between them, their temperature, and even their material composition.

Heliocentric model by Copernicus where the sun is at the center of the solar system.

Branches of Astronomy

The study of astronomy comprises so many different disciplines that it has been categorized into different branches with specific fields of study. There are five main branches in astronomy:

  • Planetary Astronomy studies the planets and their planetary systems which includes their rings and moons. It also includes the study of planet formation as well as the understanding of the atmospheric, geological, physical, biological, and chemical characteristics of these celestial objects.
  • Stellar Astronomy studies how stars are formed, their evolution, and the processes that occur during the different stages of their life cycles.
  • Solar Astronomy focuses its studies on the Sun, the center of our solar system, and the closest star to our planet. Solar astronomers study the structure of the Sun, its life cycle, as well as its interactions with other celestial bodies.
  • Observational Astronomy studies celestial bodies using sophisticated equipment like telescopes, cameras, spectrometers among other tools to make detailed observations and measurements of different objects in space.
  • Theoretical Astronomy uses mathematical or numerical models to research different astrophysical processes like radiation and dark matter.

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What is the Definition of a Star in Astronomy?

A star is an incandescent ball of gas in space. Stars produce light and heat thanks to the nuclear reactions that happen in their core where hydrogen gas undergoes a process called nuclear fusion in which two hydrogen nuclei combine to form helium. This process releases energy in the form of light and heat. A star is born the first time nuclear fusion occurs.

Stars vary greatly in size and temperature. Small stars like our sun are called dwarf stars and stars that are much larger than the average are called massive stars. The Hertzsprung-Russell diagram is a graph that organizes stars according to their brightness and temperature. Most stars are located at the region of the diagram called the main sequence, this means most stars are in the process of turning hydrogen into helium inside their cores. Stars also contain an immense amount of matter that exerts a powerful gravitational force upon other celestial bodies forming systems where planets and other objects orbit around them.

Hertzsprung-Russell diagram.

Groups of Stars

A constellation is a group of stars. Historically, stars have been grouped by many cultures to form shapes in the sky that resemble people, animals, and objects. Stars that belong to the same constellation are not necessarily close to each other or even related, they actually can be very far away from one another and have completely different origins, they were just grouped together because, from our perspective, they seemed to form a shape that we could identify. Even though the stars in a constellation are not always related, they are useful to help identify the current season by recognizing the constellations that appear in the evening sky at different times of the year. There are 88 official constellations. A group of billions of stars held together by a gravitational force is called a galaxy. Galaxies also contain dust, gases, and dark matter. There are billions of galaxies in the universe, our solar system is located in the Milky Way Galaxy.

Milky Way Galaxy.

Facts About Stars

  • Stars have a life cycle with birth and death.
  • Smaller stars live longer since they burn their fuel more slowly.
  • Massive stars have shorter life cycles because they run out of hydrogen much quicker than the smaller ones.
  • The color of a star indicates how hot it is. Red stars have the lowest temperatures followed by yellow stars. White and blue stars have the highest temperatures.
  • Stars form planetary systems.

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  • Activities
  • FAQs

Stars Word Scramble Activity

For this activity, study the scrambled letters and try to unscramble or rearrange the letters to form a word or phrase that fits the given clues. To do this, you must right-click and print this page. With a pencil and an eraser, neatly write your answers in the blank space provided.

Scrambled Words

  10. ROECLO


  1. Constellations had been used by ancient people to keep track of the time and aid in _____.
  2. __________ stars fuse hydrogen atoms to form helium atoms in their cores and constitutes 90 percent of the stars in the universe.
  3. A _____ star is a system consisting of two stars orbiting a common center of mass.
  4. _____ stars exhaust their hydrogen core at a much faster rate.
  5. Stars are not _____ and may vary greatly in age, size, brightness, and color.
  6. A _____ is an interstellar cloud that is usually created from the gas and dust thrown out by the explosion of a dying star.
  7. Our Sun would turn into a hot __________ after becoming a large red giant.
  8. A _____ is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern.
  9. Depleted supply of _____ in the star's core is indicative of its nearing death.
  10. Low-mass main sequence stars are much _____ than high-mass main sequence stars.


  10. COOLER

How do stars die?

Small and medium-sized stars bloat after burning their fuel forming a red giant star that eventually collapses forming a planetary nebula, and finally a white dwarf star. Massive stars become super red giants after burning their fuel and then undergo a supernova explosion that results in the formation of either a black hole or a neutron star.

What careers allow people to study stars?

Astronomy is the science that studies all celestial bodies including stars. Astronomy has different branches that specialize in specific topics within astronomy.

What are stars made of?

Stars are made of gases hydrogen and helium, but can also contain traces of elements like carbon, nitrogen, oxygen, and iron.

What is the study of the formation of stars?

Stellar Astronomy studies how stars are formed and the processes that occur during the different stages of their life cycle.

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