Scientists Say: Galaxy | Science News for Students

Scientists Say: Galaxy

These stellar groups can contain millions to billions of stars
Sep 9, 2019 — 6:30 am EST
an image of the Andromeda galaxy

This is a colorized image of the galaxy Andromeda. Andromeda is a spiral galaxy that neighbors the Earth’s home galaxy, the Milky Way. 

JPL-Caltech/NASA

Galaxy (noun, “GAAH-lex-ee”)

This word describes an enormous collection of stars bound together by gravity. Galaxies typically contain millions to trillions of stars, plus gas, dust and leftovers from star explosions. Scientists estimate that there are at least two trillion galaxies in the part of the universe that can be observed from Earth.

Galaxies come in lots of different shapes. Our home galaxy, the Milky Way, is a spiral galaxy. These galaxies often look like pinwheels. Elliptical galaxies are fuzzy ovals. Some galaxies are called “irregular” because they lack symmetry and can look like a bunch of blobs. Gravity also pulls galaxies together into groups called clusters. These clusters can include just a couple of galaxies or up to thousands. Really big clusters can be tens of millions of light years across or even larger.  

In some galaxies, new stars continue to form. Other galaxies have settled down and haven’t seen a new star for billions of years. At the center of many galaxies are black holes, where gravity is so strong that not even light can escape. Earlier this year, scientists got a picture of a supermassive black hole in the center of the M87 galaxy. The M87 galaxy is located in the constellation Virgo.

In a sentence

A 3-D map of stars shows that the Milky Way looks like a deformed potato chip.

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Power Words

(more about Power Words)

black hole     A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.

constellation     Patterns formed by prominent stars that lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.

elliptical     Having the shape of an ellipse, which is an oval shape.

galaxy     A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

gravity     The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

Milky Way     The galaxy in which Earth’s solar system resides.

solar system     The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

symmetry     In geometry, the property of being indistinguishable from a shifted, rotated or reflected image of the same object. For example, the letter X looks the same whether reflected in a mirror or turned upside down — two different kinds of symmetry.

trillion     A number representing a million million — or 1,000,000,000,000 — of something.

universe     The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).