18th November 2019

astronomynotes

11

What is the 21 cm line?

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In 1944 Hendrik van de Hulst predicted that the cold atomic hydrogen (H I) gas should emit a particular wavelength of radio energy from a slight energy change in the hydrogen atoms. The wavelength is 21.1 centimeters (frequency = 1420.4 MHz) so this radiation is called 21-cm line radiation.

In this manner, what is the hydrogen line?

The hydrogen line (1420.40575 MHz) is the precession frequency of neutral hydrogen atoms, the most abundant substance in space. It happens to fall in the quietest part of the radio spectrum, what's known as the Microwave Window.

What is interstellar matter?

In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space.

1

What is the hydrogen spectrum?

The emission spectrum of atomic hydrogen is divided into a number of spectral series, with wavelengths given by the Rydberg formula. These observed spectral lines are due to the electron making transitions between two energy levels in an atom.

2

What is the Lyman alpha line?

In physics, the Lyman-alpha line, sometimes written as Ly-α line, is a spectral line of hydrogen, or more generally of one-electron ions, in the Lyman series, emitted when the electron falls from the n = 2 orbital to the n = 1 orbital, where n is the principal quantum number.

3

What is the molecular cloud made of?

Giant molecular clouds are the largest inhabitants of galaxies, reaching up to 300 light years in diameter. They contain enough dense gas and dust to form hundreds of thousands of Sun-like stars. These stars are formed in the densest parts of the clouds.

4

What are interstellar gas clouds made of?

Approximately 99% of the interstellar medium is composed of interstellar gas, and of its mass, about 75% is in the form of hydrogen (either molecular or atomic), with the remaining 25% as helium.

5

What is a molecular cloud made of?

A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H₂).

6

What causes the blue color associated with reflection nebula?

Fine dust in the nebula reflects the light. The blue color is caused not only by Rigel's blue color but because the dust grains reflect blue light more efficiently than red. In astronomy, reflection nebulae are clouds of interstellar dust which might reflect the light of a nearby star or stars.

7

What is the main sequence lifetime for stars like the sun?

The Sun, a G-type star with a main sequence lifetime of ~ 10 billion years, is currently 5 billion years old – about half way through its main sequence lifetime.

8

What makes an emission nebula glow?

As these atoms fall back to lower energy states, they emit radiation. The process is similar to that of a neon light. This causes the nebula to glow. Emission nebulae tend to be red in color because of the abundance of hydrogen.

9

How is an emission nebula formed?

An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy photons emitted from a nearby hot star.

10

What is a black dwarf star?

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light.

11

What happens to a black dwarf star?

However, at less than 14 billion years old, the universe is still too young to have created any black dwarfs! A main sequence star that lacks the mass necessary to explode in a supernova will become a white dwarf, a 'dead' star that has burned through all of its hydrogen and helium fuel.

12

How hot is a black dwarf star?

Within this nebula, the hot core of the star remains—crushed to high density by gravity—as a white dwarf with temperatures over 180,000 degrees Fahrenheit (100,000 degrees Celsius). Eventually—over tens or even hundreds of billions of years—a white dwarf cools until it becomes a black dwarf, which emits no energy.

13

How long is a black hole?

It would take a whopping ~10^67 years for a black hole the mass of the Sun to evaporate, and around ~10^100 years for the largest black holes in the Universe. That may be far longer than the age of the Universe, but it's still not forever.

14

How massive is a neutron star?

Neutron stars are created when giant stars die in supernovas and their cores collapse, with the protons and electrons essentially melting into each other to form neutrons. Neutron stars are city-size stellar objects with a mass about 1.4 times that of the sun.

15

How do you detect a black hole?

The first way we detect black holes is by their gravitational influence. For example, at the center of the Milky Way , we see an empty spot where all of the stars are circling around as if they were orbiting a really dense mass. That's where the black hole is.

16

How long is the life of a neutron star?

If you're asking how long a neutron star can actually be detected as a pulsar, the answer is that in the most recent catalog of pulsars (pulsars are rotating neutron stars), the oldest ones are more than 10,000,000,000 years old (although the large majority of pulsars is between 100,000 and 300,000,000 years old.

17

Are neutron stars hot?

The temperature inside a newly formed neutron star is from around 100,000,000,000 to 1,000,000,000,000 kelvin. However, the huge number of neutrinos it emits carry away so much energy that the temperature of an isolated neutron star falls within a few years to around 1,000,000 kelvin.

18

How does a supernova turn into a black hole?

Stellar black holes form when the center of a very massive star collapses in upon itself. This collapse also causes a supernova, or an exploding star, that blasts part of the star into space. Scientists think supermassive black holes formed at the same time as the galaxy they are in.

19

What is a white hole in space?

In general relativity, a white hole is a hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it. In this sense, it is the reverse of a black hole, which can only be entered from the outside and from which matter and light cannot escape.

20

Where does a wormhole lead to?

A wormhole is a theoretical passage through space-time that could create shortcuts for long journeys across the universe. Wormholes are predicted by the theory of general relativity. But be wary: wormholes bring with them the dangers of sudden collapse, high radiation and dangerous contact with exotic matter.

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