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.
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,
Our solar system is in the milky way galaxy. if you are told that in another galaxy a planet p has a circular orbit equal to that of the Earth's (that is they both have the same radius). the masses of the suns in the 2 galaxys are Se and Sp respectively and planet p completes its orbit in half...
thanks to MARCUS.
this discussion started in S B and LQG, i thought i had better move to
a more appropriate place.
the recent discovery of a Z=10 galaxy, if confirmed means it was
was formed when our universe was only 500MYs old.
this paper discuses the discovery...
i don't understand how dark matter acts upon the rotation of a galaxy. galaxy rotation doesn't work the same way that solar system rotation does in that it's not a simple gradient of further planets rotate slower, right? nor is it further planets rotate faster, like a vinyl record...i take it...
I bought a nexstar 114gt a few months ago, and I have been unable to use the auto align features, which would allow me to use the tracking and goto features on the telescope. I am not certain if the telescope itself is flawed, or if I am simply doing something wrong. I sent two emails to...
Roser Pello and her colleagues at the MidiPyrenees observatory
have reported finding a z=10 galaxy
this galaxy is currently 31.5 billion light years from us
and is receding from us at 2.3 times the speed of light
for more details see "the most distant object thread"
especially links...
This is my first post, so I am going to get straight into a double barreled question that has been bugging me for a while.
(1) How long does it take for stars to conglomerate around a general loci, in order to form a proto-galaxy. ?
I ask this in the light of the most recent Hubble images...
Dear Sir,
Please Find the story at the link http://www.internationalreporter.net/scripts/headDetails.asp?id=108, which has a story ,also given under. This could be an eye opener to the scientific community.
We definitely seek your response regarding this.
Thanx..
Arif
FARTHEST KNOWN...
Dear Sir,
Please Find the story at the link http://www.internationalreporter.net/scripts/headDetails.asp?id=108 which has a story ,also given under. This could be an eye opener to the scientific community.
We definitely seek your response regarding this.
Thanx..
Arif
FARTHEST...
I hadnt heard of GW recoil imparting velocity to a black hole.
http://www.arxiv.org/astro-ph/0402056
"How Black Holes Get Their Kicks..."
It's an article by three people, from Cornell, MIT, and Chicago.
When two black holes spiral in and merge, linear momentum is carried off...
So, this doesn't fit the current model, right?
http://heraldsun.news.com.au/common/story_page/0,5478,8344220%255E662,00.html
Nasa: http://www.gsfc.nasa.gov/topstory/2004/0107filament.html
A newly released mosaic image from Hubble, with 4 billion pixels, covering a square of sky about 1/2o wide, with an angular resolution of 0.05", and a depth of 24 or 25 mag.
"Not only does the GEMS study cover a large area at high resolution; it is also a three-dimensional map. Of the 60,000...
First of all let me say Hello! as this is my first post. I am a biologist, and have been reading over the very basics of SR for a little while and have a lot of questions on the implications of the theory.
I had been asking my questions primarily at a different forum, one that wasn't...
I've been reading some books on Galactic Structures and Galactic Evolutions, evidence from rotation curves of stars near the centre of our Milky Way and also M31 suggest that there is a black hole in the centre. But is this true for all galaxies regardless of their Hubble Type? is there a...
"The nearest galaxy to our own Milky Way has been revealed. It is so close that the Milky Way is gradually consuming it by pulling in its stars. But it will be few billion years before it is entirely swallowed up.
The previously unknown galaxy lies about 25,000 light years from Earth and...
Has anybody here read Douglas Adams' The Hitchhiker's Guide to the Galaxy ?
I just started the series and am only about 56 pages into it. The whole book is hilarious so far. It's so full of puns, humor, and whim. Has anyone else noticed some subtle parodies to Asimov's Foundation trilogy? Like...
Greetings !
I thought you may like to know that our galaxy -
the Milky Way, apparently has a small settelite
galaxy at relativly close proximity.
Here's the story:
http://www.spacedaily.com/news/milkyway-02a.html
Live long and prosper.
Greetings !
I just read Saint's post in the "moving galaxy"
thread and though I knew the answer to that, I do
not believe I have so far read somewhere an
explanation for the existence of the galactic
cluster sheets.
So, is there a known and recognized reason ?
(My guesses :
1. The...
I had come across some scientific articles that Galaxy is actually moving across the space!
Is that movement due to initial explosion of the universe? The big-bang?
When thing moves, it has energy 1/2M.V^2 ,
if the galaxy which is so huge is moving, from where does it get the kinetic...
"Skinny" galaxy harbors massive black hole at core
http://www.berkeley.edu/news/media/releases/2003/04/10_skinny.shtml
What kind of changes to our "rule of thumb" can we expect from this finding?
hi,was the formation of our galaxy inevitable after the BB or could there have been a different outcome, if the former is true then that implies that information or rules were stored pre BB, if the later is true what other outcome could arise.
wolram.