Galaxies VSI

The age of the Universe is determined by studying the largest things using general relativity. The age of star comes from studying the smallest using quantum mechanics. The consistency between the two is a profound result.

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Galaxies

1. How Many Stars Are There in the Universe?

The number of stars in the observable universe is an estimate since the universe is vast and expanding. Current scientific estimates suggest that there are roughly 1 to 10 trillion galaxies in the observable universe, and each galaxy contains millions to trillions of stars. A widely accepted estimate places the number of stars in the observable universe at around \(10^{23}\) stars.

2. How Many Galaxies Are There in the Universe?

Based on data from the Hubble Space Telescope and other astronomical observations, there are estimated to be about 2 trillion galaxies in the observable universe. This number has been revised upwards from earlier estimates as telescopes become more powerful and capable of detecting faint and distant galaxies.

3. How Many Stars Are in the Average Galaxy?

Galaxies vary significantly in size and the number of stars they contain. However, on average, a typical galaxy is thought to contain between 100 billion and 400 billion stars. This average includes large galaxies like the Milky Way as well as smaller ones like dwarf galaxies.

4. What Are the Different Types of Galaxies?

Galaxies are generally classified into four main types based on their appearance:

1. Spiral Galaxies

• Structure: Characterized by a flat, rotating disk with spiral arms extending outwards, a central bulge, and a surrounding halo.
• Examples: The Milky Way, Andromeda.
• Relative Frequency: About 70% of known galaxies.
• Star Count: Typically contains 100 billion to 400 billion stars.

2. Elliptical Galaxies

• Structure: Ellipsoidal shape, with little or no internal structure, lacking significant spiral arms or a defined disk.
• Examples: M87, Centaurus A.
• Relative Frequency: About 20% of known galaxies.
• Star Count: Can range from a few million to over one trillion stars, depending on the size.

3. Irregular Galaxies

• Structure: No regular shape; chaotic appearance, often due to gravitational interactions or collisions.
• Examples: Large Magellanic Cloud, Small Magellanic Cloud.
• Relative Frequency: About 5% of known galaxies.
• Star Count: Typically contains fewer stars, around 1 billion to 10 billion stars.

4. Lenticular Galaxies

• Structure: Has a central bulge and a disk but lacks spiral arms. It’s considered a transitional type between spiral and elliptical galaxies.
• Examples: NGC 5866.
• Relative Frequency: About 5% of known galaxies.
• Star Count: Typically contains 10 billion to 100 billion stars.

Summary of Galaxy Types and Their Characteristics:

Galaxy Type	Relative Frequency	Average Star Count
Spiral	~70%	100 billion to 400 billion
Elliptical	~20%	10 million to 1 trillion
Irregular	~5%	1 billion to 10 billion
Lenticular	~5%	10 billion to 100 billion

This classification is based on observable characteristics and is subject to refinement as our observational technology and understanding of the universe improve.

Baryonic Matter

The baryonic mass of the universe, which refers to the ordinary matter made up of protons, neutrons, and electrons, is distributed among various components. Here’s a breakdown of how the baryonic mass is divided:

1. Stars

• Percentage of Baryonic Mass: Approximately 5-10%.
• Description: Stars, including those in galaxies and globular clusters, contain a significant portion of the universe’s baryonic mass. This includes main sequence stars, giants, dwarfs, and neutron stars.

2. Interstellar and Intergalactic Gas

• Percentage of Baryonic Mass: Approximately 60%.
• Description: The majority of baryonic matter in the universe is found in the form of gas, primarily hydrogen and helium. This gas exists both within galaxies (interstellar medium) and between galaxies (intergalactic medium). A large portion of this gas is in a hot, diffuse state, such as in the circumgalactic medium or the warm-hot intergalactic medium (WHIM).

3. Intracluster Medium (ICM)

• Percentage of Baryonic Mass: Approximately 20%.
• Description: The intracluster medium consists of hot, ionized gas that fills the space between galaxies in galaxy clusters. This gas is very hot, emitting X-rays, and contains a significant fraction of the universe’s baryonic matter.

4. Dust

• Percentage of Baryonic Mass: Less than 0.1%.
• Description: Cosmic dust is made up of tiny solid particles composed of elements like carbon, silicon, and metals. It plays a crucial role in star formation and the thermal regulation of the interstellar medium but accounts for only a tiny fraction of the total baryonic mass.

5. Black Holes

• Percentage of Baryonic Mass: Approximately 0.01-0.05%.
• Description: Supermassive black holes at the centers of galaxies and stellar-mass black holes formed from collapsed stars contain a small portion of the baryonic mass. Despite their gravitational influence, black holes represent a minor fraction of the total baryonic mass.

6. Planets and Other Objects

• Percentage of Baryonic Mass: Less than 0.1%.
• Description: Planets, moons, asteroids, comets, and other small objects contribute a negligible amount to the total baryonic mass.

7. Compact Stellar Remnants (White Dwarfs, Neutron Stars)

• Percentage of Baryonic Mass: Approximately ~0.5%.
• Description: These remnants are the end states of stars and include white dwarfs and neutron stars. While small in volume, they contribute a measurable fraction to the baryonic mass.

Component	Approximate Percentage of Baryonic Mass
Interstellar and Intergalactic Gas	~60%
Intracluster Medium (ICM)	~20%
Stars	~7%
Compact Stellar Remnants	~0.5%
Dust	~0.1%
Black Holes	~0.01-0.05%
Planets and Other Objects	< 0.01%
Unaccounted/Uncertain	~12%

ICM vs Dust

The Intracluster Medium (ICM) and dust are two distinct components of the baryonic matter in the universe, differing significantly in their composition, location, and role in the cosmos. Here’s a breakdown of the differences:

Intracluster Medium (ICM)

1. Location:

• The ICM is found within galaxy clusters, filling the space between galaxies in these clusters.

2. Composition:

• The ICM consists primarily of hot, ionized gas, mostly hydrogen and helium, with trace amounts of heavier elements. The gas is so hot (millions of degrees Kelvin) that it is in a plasma state and emits X-rays.

3. Density:

• The ICM is very diffuse, with a low particle density (about 10 to 1000 particles per cubic meter), but because it spans vast volumes, it contains a significant fraction of the cluster’s baryonic mass.

4. Temperature:

• Extremely high temperatures, ranging from about 10 million to 100 million Kelvin. The gas is heated by the gravitational potential of the galaxy cluster.

5. Role:

• The ICM is a key component in understanding galaxy cluster dynamics and cosmology. It interacts with galaxies via ram pressure stripping and can be used to trace the mass of galaxy clusters through its X-ray emission.

6. Observational Characteristics:

• The ICM emits strongly in X-rays due to the high temperature of the plasma. This X-ray emission is one of the primary ways the ICM is detected and studied.

Dust

1. Location:

• Dust is found within galaxies, primarily in the interstellar medium (ISM). It can also be found in smaller quantities in the circumgalactic medium (CGM) and, to a much lesser extent, in the intergalactic medium (IGM).

2. Composition:

• Dust consists of small solid particles, primarily silicates, carbonaceous compounds, and sometimes ice. These particles are typically only a few micrometers in size.

3. Density:

• Dust is extremely sparse compared to gas, making up only a tiny fraction of the baryonic mass in the universe. In the ISM, dust particles are mixed with gas, usually at a ratio of about 1 part dust to 100 parts gas by mass.

4. Temperature:

• Dust particles are much cooler than the ICM, with temperatures typically ranging from 10 to 100 Kelvin. Dust can absorb starlight and re-radiate it in the infrared, making it detectable via infrared telescopes.

5. Role:

• Dust plays a crucial role in star formation by cooling gas clouds, which can then collapse under gravity to form stars. It also scatters and absorbs light, affecting the appearance of galaxies and the interstellar medium.

6. Observational Characteristics:

• Dust is observed primarily through its effects on starlight (extinction and reddening) and its infrared emission. It can obscure our view of certain parts of galaxies, including regions of active star formation.

Summary of Key Differences:

Feature	Intracluster Medium (ICM)	Dust
Location	Between galaxies in galaxy clusters	Within galaxies, in the interstellar medium
Composition	Hot, ionized gas (hydrogen, helium)	Solid particles (silicates, carbon, ice)
Density	Very low, but significant over large volumes	Extremely sparse, mixed with gas
Temperature	Extremely hot (10 million to 100 million K)	Cold (10 to 100 K)
Role	Dominates the mass in galaxy clusters	Crucial in star formation and affects light transmission
Observational Wavelength	X-rays	Infrared, visible light scattering

These differences highlight the distinct roles that the ICM and dust play in the universe, with the ICM dominating in the large-scale structure of galaxy clusters, while dust has a more localized impact within galaxies.

Deets

• John Gribbin
• Volume 182
• Published 2008