What is meant by Galactic Halo?

What is a Galactic Halo?

• Galactic halo is a nearly spherical volume of thinly scattered stars, globular clusters of stars, and tenuous gas observed surrounding spiral galaxies, including the Milky Way.
• Galactic halo is dominated by invisible dark matter, whose presence is only measurable through the gravity that it exerts.
• Only about 1% of a galaxy’s stellar mass resides in its halo, and due to this low luminosity, the observation of halos in other galaxies is extremely difficult.

What is Stellar Halo?

• It is the visible portion of a galactic halo. 
• It is composed of globular clusters and field stars which have high space velocities that can take them far out of the galactic disk.
• Stellar halos may hold some of the best-preserved fossils of the formation history of galaxies.

What are Globular clusters:

• They are stable, tightly bound clusters of tens of thousands to millions of stars. 
• They are associated with all types of galaxies.
• They are more densely populated, with populations ranging from tens of thousands to millions of stars. 
• The intense gravitational attraction between the closely packed stars gives globular clusters their regular, spherical shape.

What is meant by a field star?

• Stars that are not connected with an astronomical object being studied but which happen to appear in the same field of view when the object is observed through a telescope. Typical examples of field stars are those that appear in the foreground of a telescopic image of a distant galaxy.
• A field star is not part of any stellar cluster such as a globular cluster or open cluster.

 

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Q1) What is a dark matter?

Over 80% of all matter in the universe is made up of  dark matter. By dark we mean that it does not emit any form of electromagnetic radiation. The existence of Dark Matter is inferred indirectly by its gravitational effect.

Source: Astronomers discover Milky Way galaxy's most-distant stars