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## How many AU is the nearest Galaxy from the milkyway?

George C.
Featured 6 months ago

It depends what you mean by nearest galaxy, but the conversion factor between light years and AU is about $63000$

#### Explanation:

Let's work out a conversion factor between light years and AU first.

The Sun is roughly $93000000$ miles from the Earth and light travels at about $186000$ miles per second.

So $1$ AU is about $500$ light seconds.

There are $86400$ seconds in a day and about $365.25$ days per year, hence:

$86400 \cdot 365.25 = 31557600 \text{ }$ seconds per year.

Dividing this by $500$ we arrive at:

$\frac{31557600}{500} = 63115.2 \approx 63000 \text{ }$ AU per light year.

$\textcolor{w h i t e}{}$
How far away is the nearest galaxy from the Milky Way?

The very nearest galaxy would be one of the small satellite galaxies.

There are about $50$ galaxies within about $1.4$ million light years of the Milky Way. The closest are about $1000$ light years from the edge, which by our reckoning would be about $63000000 = 6.3 \times {10}^{7}$ AU from the edge.

Perhaps of more interest is the distance to the Andromeda galaxy, our largest neighbour in the Local Group. This is about $2.5$ million light years away, which would make it about $160$ billion AU away.

## If there is a supermassive black hole at the centre of our galaxy, why is our galaxy so stable?

Oscar L.
Featured 5 months ago

Because a black hole's gravity is all-powerful only within a limited amount of space. Most of our galaxy is so far away from the hole that things are safe even if they move relatively slowly.

#### Explanation:

What makes a black hole able to capture everything, including light, is when all its mass can simultaneously exert gravity in one direction at close range.

To understand this, suppose you could go inside the Sun and get within 10 km of the center. Most of the Sun's mass is then equally distributed in all directions around you, and thus can't pull you in one direction. Only the small part of the Sun's mass in a ball concentric with the Sun and lying below you can exert a net force towards the center. The Sun's gravity becones weaker not stronger as you get close to the center, once you get inside most of its mass.

But suppose that the mass, instead of being in a big fluffy ball a million km or so across, were concentrated in one point so you could not get inside any of the mass. Now at a distance of 10 km all that mass is pulling you onwards, not just a tiny bit of "net" mass. You experience far stronger gravity than what you find 10 km from the actual center of the Sun, or hundreds of thousands of km away at the surface of the real Sun. And it keeps strengthening as you go within 8 km, 6 km, 5, 4 , ...

At 3 km away from the concentrated solar mass, with all the mass still below you, gravity becomes so great that you have no chance to escape, not even light can escape anymore, never mind that light could have escaped from the center if there had been a big, fluffy Sun with its mass all spread out. That 3 km is called the Schwarzschild radius corresponding to one solar mass. It's the range within which a mass must be concentrated to get that all-powerful gravity. General relativity tells us that the Schwarzschild radius is directly proportional to the amount of mass.

How does this relate to the center of our galaxy? Here we see a lot of concentrated mass, maybe four million times the mass of the Sun. The Schwarzschild radius is then four million times that of one solar mass, thus 12 million km. That is a lot of space ... or is it? The volume inside the Schwarzschild radius would not even engulf the orbit of Mercury, let alone a whole galaxy over a hundred thousand light years across.

So, only a very tiny part of our galaxy is exposed to the full gravitational power of that central black hole. Most of it has a much weaker, more "ordinary" gravity around it.

## What started the Big Bang? What was there before it? What is some evidence for it? Why do some people not agree?

Ricardo A.
Featured 5 months ago

#### Explanation:

We have, however many hypotheses that could explain it, such as string theory or m-theory, the problem we have right now is that all the predictions these ideas make about the universe are impossible to test with our current technology. You see, all scientific models make predictions and we can prove or disprove them by going out and making observations that confirm or deny that model.

• What was there before it?

Same as above, we have models that could explain it, but the observations are beyond our tech so far.

• What is some evidence for it?

In the 1920s Edwin Hubble (the telescope was named after him) discovered that all galaxies are flying away from us, and the further away a galaxy is from us the faster it is going away from us. This proved the Universe was expanding. Meaning it must have been smaller early on. This begat the Big Bang Theory.
There are many cosmological observations that corroborate this, and one of the best, which was predicted by the theory itself, is the Cosmic Microwave Background.

• Why do some people not agree?

Overall the school system has done a poor job of communicating the science to the people. The media also doesn't help as it is sensationalist and misrepresents the issues most of the time, leading to people distrusting science. Religion also plays a part in it as science disproves some of the stuff written in their holy books, so they (true to their doctrines) truest blind faith over knowledge.

## Why is the Hubble Space Telescope Important?

Gió
Featured 5 months ago

I would say because of the technical achievement in itself and for the fact that is allows the observation of celestial bodies without the interference of the Earth's atmosphere.

#### Explanation:

We can consider the incredible achievement of actually sending such complex piece of hardware in orbit (and operate it) as one of the most important successes in the field of engineering and automation. This testifies the high level of expertise and accuracy reached by humans in building very complex machines.

Another relevant achievement (in particularly for Astronomy) of the telescope is the possibly to show us distant objects without the interference (refraction and diffraction) of our atmosphere that changes density with height and distorts and blurs our observations (also reducing the Resolution of our observations).

[An example of the Hubble Space Telescope’s superior resolution compared with that of a standard ground-based telescope: (left) a distant, peculiar interacting galaxy imaged with the Subaru telescope on Mauna Kea; (right) the same object imaged with Hubble. Subaru (8 m) telescope image courtesy of National Astronomical Observatory of Japan; Hubble (2.4 m) image courtesy of STScI/NASA.]

## What are the similarities and differences between pulsars, quasars, white dwarfs, neutron stars and black holes?

Phillip E.
Featured 3 months ago

Pulsars, white dwarfs, neutron star and black holes are the remains of dead stars, quasars are powered by black holes.

#### Explanation:

When a star less than about 8 solar masses runs out of hydrogen and helium fuel, its core isn't hot enough to start carbon fusion. The core which consists of mainly carbon and oxygen collapses under gravity to form a white dwarf. Gravitational collapse is stopped by electron degeneracy pressure.

If the star is larger than about 8 solar masses it is able to fuse heavier elements up to iron. As iron fusion required energy rather than releasing it the fusion reactions stop and the stellar core collapses under gravity. It the core is more massive than the Chandrasekhar limit of 1.44 solar masses gravity overcomes electron degeneracy pressure atoms can no longer exist. Protons become neutrons and large numbers of neutrinos are emitted causing a supernova explosion. The star's core become a neutron star.

If a neutron star is spinning and has a strong magnetic field it emits radiation. As it spins at a precise rate the beam of radiation its the Earth periodically with a period of milliseconds to seconds. This is a pulsar.

If the stellar core is more than about 4 solar masses gravity overcomes neutron degeneracy pressure. Once the core collapses below its Schwarzschild radius, spacetime is curved to the point where not even light can escape. This is a black hole.

Most large galaxies have a supermassive black hole at their centres. These are in excess on hundreds of thousands of solar masses. If there is a good supply of gas and dust in the vicinity of a supermassive black hole it forms an accretion disc of material falling into the black hole. Material falling into the accretion disc gets superheated by friction and gravity to the point where it emits huge amounts of energy. This is a quasar.

So, all are similar in that they are formed from the remains of dying stars. Pulsars are a type of neutron star. Neutron stars and black holes behave similarly. The main difference between these objects is mass.

## How do we know that galaxies farther away from us are moving faster than nearby galaxies?

Dwight
Featured 1 month ago

Astronomers use the Doppler Shift to determine the speed at whih a source of light is moving. The faster the source moves, the greater the shift in the colour of the light we observe (compared to the colour of a stationary source).

#### Explanation:

Anyone who has listened to a train approaching and passing on a track has been aware of the change in sound pitch that occurs. This effect is known as the Doppler Shift, and is well understood for all types of waves, including sound and light.

The image is meant to show that if the wave source is moving toward us, the waves will be compressed ahead of the source, and a high frequency is heard. Behind the source, the observed pitch is lower than that of a stationary source. The faster the source moves, the greater the change in pitch that is observed.

With light, the effect is not in pitch, but colour. If a source of light moves toward an observer, the waves will be shorter in wavelength and higher in frequency. The light will appear to be shifted toward the blue end of the spectrum. If the source is moving awat from the observer, the shift in frequency causes the colour to appear more red. As in the case of sound, the faster the source moves, the greater the change in colour that is observed. This is the red shift that is seen in all galaxies.

So, to finally answer your question, when we measure the light from more distant galaxies, we routinely note larger shifts in the colour of the light they emit. This tells us they are moving at greater speed than the galaxies that are "close by".

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