A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant galaxy observed. It has a comoving distance of 32 billion light-years from Earth, and is seen as it existed just 400 million years after the Big Bang.
In 2021, data from NASA's New Horizons space probe was used to revise the previous estimate of 2 trillion galaxies down to roughly 200 billion galaxies (2×1011). This followed a 2016 estimate that there were two trillion (2×1012) or more galaxies in the observable universe, overall, as many as an estimated 1×1024 stars (more stars than all the grains of sand on planet Earth). Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 ly) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million ly.)
The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which it dominates along with Andromeda Galaxy. The group is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea.
Hello. I'm plowing through Linda Sparke's "Galaxies in the Universe: An Introduction". I'm enjoying it quite well and learning a lot. But then I get to Equation 1.18 in the book and am a little confused because it uses variables not previously introduced.
Not sure if I can do it here but...
Dark Galaxy found!?
You know, I've always been secretly hoping that dark matter was an aberration of our observations or calculations, some value to be tweaked or some normalizing factor to be applied thing that was making galaxies turn faster than formulae predicted. I just couldn't believe...
The Andromeda glaxaxy is two million light-years from earth, measured in the common rest frame of the Earth and Andromeda. Suppose you took a fast spaceship to Andromeda, so it got you there in 50 years measured on the ship. If you sent a radio message home as soon as you reached Andromeda, how...
I decided to post a new thread to distinguish from my other post
The paradox is as this: suppose that 10 billions years ago a guy was born ( let's call him "Universeguy"), in a place near the Earth (because obviously Earth doesn't exist yet), first, he was a lucky guy , he discover how to...
Why are the planets in our solar system spread out in a plane instead of being more randomly spread around the Sun? And why is our galaxy in the shape of a plane instead of all the stars in our galaxy being more randomly spread around the middle of our galaxy?
Thanks,
The post that held the link to this paper has been moved. I would like to see comments on this paper, including impressions of the mechanism responsible for the interaction of the quasar and the associated galaxy.
http://arxiv.org/ftp/astro-ph/papers/0509/0509433.pdf
I objected to the...
This topic brings up the Fermi Paradox, once again. I just want to hear the boards opinion on this...should the galaxy have been colonized by now? and a decent portion of it has been colonized would we able to "see" it?
New pics. These are a month old, but I forgot to upload them. They are from Sept 29. That was the first night it got really cold and I drove to the Poconos to set up shop, so while the sky was very clear and very dark, it was also very windy.
First is another shot at the Andromeda Galaxy...
for the following question:
In its own frame of reference, a proton takes 5min to cross the Milky Way galaxy, which is about 10^5 light-years in diameter.
What is the approximate energy of the proton in electronvolts?
my problem:
v*5*60=(10^5)*3*(10^8)*[1-(v^2/c^2)]
however, i can't...
This time the news is that on 21 September this paper was accepted for publication in Astrophysical Journal. It will appear in January.
http://www.arxiv.org/abs/astro-ph/0506370
Galaxy Rotation Curves Without Non-Baryonic Dark Matter
J. R. Brownstein, J. W. Moffat
43 pages, 7 figures, 4...
Claims universe not expanding:
http://www.arxiv.org/abs/astro-ph/0509611
Another recent article about redshifts being apparently wrong:
Research on candidates for non-cosmological redshifts
"The paradox of apparent optical associations of galaxies with very different redshifts, the...
I know that the velocity of our galaxy towards Andromeda is approximately 300,000km/s, but how quickly does this velocity change? Or can we tell given our relatively short time observing the sky?
Hi, i have a question, if we imaging that the universe is static and not expanding, what observation would be made in regards to red shifts in galaxy spectra?
Something important to think about:
If it takes us between 225 and 240 millions years to make one Galaxy orbit wouldn’t it be prudent to associate magnetic polar shift, global warming, and ice-ages to where the sun was located when these events occurred? Were we close to or farther away...
a supernova can out light a entire galaxy. I've read it can give out about 10^44J of energy is there anyone of you who now the amount of energy released mor exacly or even better know how to calculate it? i love calculating stuff.
I was reading on Space.com about our galaxy and it said that there is a bar of stars, mostly old and red, at the center and at a 45 degree angle from the plane the rest of us are on. In addition to this there is a super-massive black hole. How can the BH be there and the bar of stars, let...
I am reading about galaxy surveys.
I want to know what is shot noise ? how we estimate it
into real and Fourier space ? I am basically reading
astro-ph/0503603 & astro-ph/0503604. I am not able to solve exercise 4, 5, 6.
I will be thankful, if anybody can provide me some reference.
The CDM model predicts the existence of lots of dark halos without star formation. These are halos which did not attract enough baryons in order to set some kind of gravitational instability, which could lead to the further collapse of the baryons and to star formation (see, e.g...
I recently came across an argument that stated that Earth's nighttime view pretty much always faces the outer rim of our galaxy, while the day time view is facing right towards the center, which of course we don't see due to sunlight. There was refrenced a few indian tribes in s.america who trek...
If there a massive black hole at the center of each galaxy then; near the center of our galaxy, stars should be revolving the BH at very high speeds, hence we should be able to note their revolutions around the BH in months, days or even hours.
And hence we should also be able to see some of...
A Cepheid variable in a nearby galaxy looks 10^6 times fainter than an identical Cepheid in the Milky Way. The Cepheid in the Milky Way is 1000 parsecs away. How far away is the nearby galaxy?
I know this formula
v = hd
but how does that help me?
i'm lost...
Hey guys
For those who have seen HHGG, that sweet white Nokia cellphone that Arthur Dent has, does it exist? If so, does anyone know the model #?
Thank you
Aychamo
As you can read in http://pinot.phys.uvic.ca/~jfn/mywebpage/gxform.html, recent simulations show that the accepted model of dissipational collapse for the formation of galaxy disks, leads to an excessive loss of angular momentum in the disk when mergers are considered. What are the current...
I have another exam question, which I'm having some slight problems with...
"In a particular galaxy the velocity curve is constant over a large range of radii, and equal 300 km/sec at a particular radius of 10kpc in this range. Estimate the mass inside this radius."
Can I use centrifugal...
http://ernie.ecs.soton.ac.uk/opcit/cgi-bin/pdf?id=oai%3AarXiv%2Eorg%3Aastro%2Dph%2F0402319
Reionization seems to be a fundamental process to the formation of our universe, but when did it occur? I have seen papers state z=6, but this seems
to be a lowest limit, as this paper suggests a galaxy...
http://www.physicsweb.org/articles/news/9/3/2/1
title: "Galaxies break new records"
Last year Roser Pello's team at the Pyrennees Observatory thought they had found a 10 billion year old galaxy IIRC, but this was later challenged [CORRECTION: hellfire reminded me that Roser thought she had...
Is the Andromeda Galaxy going to collide with the Milky Way?
I've heard that the Andromeda Galaxy is going to collide with the Milky Way someday. I understand how its radial velocity relative with respect to the Milky Way is determined through its blueshift, but how do we know its tangental...
so i heard that a galaxy was headed at us at like 100 times faster than a bullet. so what happens when it gets to us. Earth would be dead by then. mankind with it probably. but what happens to the galaxy?
good day,
well the coming fall I'm entering the astronomy faculty
and i started from now to study the NGC and M objects...
while i was searching for the names and numbers on the web..
i have found several descriptions, which i didn't understand:
1) when they display the distance in LY, is...
Is our galaxy in the centre of the universe?
If it is not, how can the age of galaxies be determined?
how can data from the CMBR be correct?
IF our position in the universe is unknown, (we could be near the event horizon)
how would this effect our knowledge of cosmology?
Hi!
Let's say that astronomers observe some galaxy or something several million lightyears away and then come to conclusions about that object. But since the light from there has been on the way for millions or even billions of years, then is it correct, that they are actually seeing the...
This is amazing, I haven't heard of any merger of two galaxy clusters until now.
http://www.newscientist.com/news/news.jsp?id=ns99996443
"One of the hottest, most energetic mergers of two colossal galaxy clusters has been imaged in exquisite detail by an X-ray observatory in space...
http://www.msfc.nasa.gov/news/news/releases/2004/04-231.html , a Chadra PR, goes on to say: "A nearby galaxy cluster is facing an intergalactic headwind as it is pulled by an underlying superstructure of dark matter, according to new evidence from NASA's Chandra X-ray Observatory. Astronomers...
Good Paper if anyone is interested:http://uk.arxiv.org/abs/gr-qc/0408008
The paper deals with some aspects for Entropy Conservation for Dynamical Blackholes, from the abstract :An effective gravitational-radiation energy tensor is obtained, providing measures of both ingoing and outgoing...
How can we see the x-ray emission from the center of our galaxy? I thought that the intersteller dust extinguished practically all of the visible light from that direction, which is why we can't "see" the bulge. How then, can even shorter wavelength photons manage to get through? I thought...
several months back some PF people were discussing
the finding of a z = 10 galaxy by a French team led by
Roser Pello
Now three Anglo-Saxons at the Imperial College have
reluctantly expressed doubts: they have tried to find the
reported Lyman Alpha line and did not see it.
S. J...
Largest Galaxy:
The central galaxy of the Abell 2029 galaxy cluster, 1,070 million light-years from Earth, has a major diameter of 5.6 million light years - 60 times the diameter of the Milky Way Galaxy.
Reference:
Guinness World Records 2000 - Millennium Edition
why aren't we spending more money on developing nuclear-powered rockets?
an interesting article about nuclear rocket propulsion
"...Nuclear-reactor rockets, like the ones that would be used in the Bimodal Nuclear Thermal Rocket, conduct nuclear fission reactions -- the same kind employed...
Would you please tell me what is in there ? I mean what substances are existing ? Where can i find out some information about this ?
Thank you very much,