Nebula: Nebula is an interstellar cloud in which dust, hydrogen, helium and other ionized gases are gathered. Originally a general term in astronomy, it refers to any astronomical diffuse celestial body.
Galaxy: A galaxy in a broad sense refers to an operating system consisting of countless star systems (including stars themselves) and dust (such as nebulae). Referring to the Milky Way, it is a huge galaxy containing stars, interstellar gas, cosmic dust and dark matter, and bound by gravity.
Cluster: A cluster refers to a group of stars with more than 10 stars and their physical relationship (gravity). A cluster with loose structure and irregular shape composed of ten to hundreds of thousands of stars is called an open cluster.
They are mainly distributed in galactic plane, so they are also called galactic clusters, which are mainly composed of blue giant, such as the Pleiades cluster (also known as the Pleiades cluster); Tens of thousands to hundreds of thousands of stars are like a circle as a whole, and clusters with dense centers are called globular clusters.
2, the classification is different
Nebulae: In terms of luminous properties, they can be divided into emission nebulae excited by high-temperature shining stars at or near the center (earlier than B 1), reflection nebulae that emit light by reflecting and scattering the radiation of low-temperature shining stars (later than B 1), and dark nebulae that partially or completely block the background stars (such as Orion's head).
The first two are collectively called bright nebulae, and those whose brightness changes with time are called variable nebulae. The only difference between reflection nebula and dark nebula lies in the relative positions of stars, nebulae and observers.
Galaxies: Galaxies are mainly divided into three categories: elliptical galaxies, spiral galaxies and irregular galaxies. A more explicit and extensive description of galaxy types will be described in the entries of Haber sequence.
Because Haber's sequence is based on visual patterns, he may miss some important features of galaxies, such as star formation rate (at the core of starburst galaxies or active galaxies). Lens galaxy is a kind of galaxy between elliptical galaxy and spiral galaxy.
Cluster: An open cluster consisting of a dozen to thousands of stars, with loose structure and irregular shape, which is mainly distributed on the galactic plane, so it is also called a cluster of galaxies, and it is mainly composed of blue giant, such as the Pleiades Cluster (also known as the Pleiades Cluster).
Globular clusters consist of tens of thousands to hundreds of thousands of stars, just like circular clusters with dense centers.
3. The discovery time is different
Nebula:1On the evening of August 28th, 758, a French astronomy enthusiast named messier suddenly found a cloud-like patch with unchanged positions among stars during the observation process of searching for comets.
Galaxy: 16 10 years, Galileo used his telescope to study the bright band in the sky, which was then called the Milky Way, and found that it was formed by a large number of dim stars.
Cluster: In 677, astronomer Harley discovered this cluster and mistook it for a star. Because although you can see it directly with the naked eye, you can't tell the stars reunited inside it. People gave it a Greek letter and called it Omega Centauri.
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