what are large groups of stars, gas, and dust held together by gravity?
A galaxy.
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Large groups of stars, gas, and dust held together by gravity are called galaxies. Galaxies are massive systems that contain billions to trillions of stars, along with different types of interstellar matter such as gas, dust, and other celestial objects.
To understand this concept, we need to know how astronomers study and classify galaxies. There are a few methods:
1. Observation: Astronomers use powerful telescopes, both ground-based and space-based, to observe and study galaxies. These telescopes capture light across the electromagnetic spectrum, allowing scientists to examine different features and characteristics of galaxies.
2. Photometry: Scientists measure the brightness of galaxies in different wavelengths of light to obtain information about their size, mass, and composition. For this, they use specialized instruments like photometers or CCD cameras.
3. Spectroscopy: By using spectrographs, astronomers separate the light emitted by galaxies into different wavelengths. This helps them analyze the chemical composition, temperature, and motion of objects within galaxies.
4. Modeling: Astronomers also employ computer simulations and mathematical models to understand the formation and evolution of galaxies. These models take into account factors such as gravity, gas dynamics, and the interplay of various astrophysical processes.
By combining these methods, astronomers can identify and classify different types of galaxies, such as spiral galaxies, elliptical galaxies, irregular galaxies, and more. The study of galaxies is ongoing, and new discoveries and insights continue to expand our understanding of these fascinating structures.