What is Milky way: Definition and 183 Discussions

The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλακτικός κύκλος (galaktikos kýklos, "milky circle"). From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a barred spiral galaxy with an estimated visible diameter of 100,000–200,000 light-years. Recent simulations suggest that a dark matter disk, also containing some visible stars, may extend up to a diameter of almost 2 million light-years. The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.It is estimated to contain 100–400 billion stars and at least that number of planets. The Solar System is located at a radius of about 27,000 light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, a supermassive black hole of 4.100 (± 0.034) million solar masses. Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much (about 90%) of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter". The rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.

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  1. jedishrfu

    B How black hole Sgr-A* Was Found

    https://www.astronomy.com/science/50-years-ago-a-first-hint-of-the-beast-lurking-in-the-heart-of-the-milky-way/
  2. Vanadium 50

    I Where are the black holes?

    Where should the nearest black hole be? Something like 0.1% of stars end up as BH's, so that suggests about 100 million in the Milky Way. The easiest thing to do, instead of a complicated geometry problem, is to recognize that the cube root of 0.1% is 0.1, so that we expect BH's to have...
  3. M

    I Inquiry Regarding the Determination of the Galactic Disk's Outer Bound

    I have recently read research papers that mention that disk stars have been found up to distances as far as 25 kpc from the milky way galactic center and possibly even up to distances as far as 31 kpc. I was wondering if there is a particular distance (or distance range) observed where the...
  4. T

    I Star cluster in the Milky Way appears to be as old as the Universe

    https://www.sciencenews.org/article/star-cluster-milky-way-old-universe https://doi.org/10.48550/arXiv.2306.02180 Cheers, Tom
  5. S

    I If GRB-221009A had been in Milky Way, would we have been fried?

    https://en.wikipedia.org/wiki/GRB_221009A
  6. Al-Layth

    I Measuring the Velocities of Astronomical Objects in the Milky Way

    I am looking for the most relied upon studies that measured the motions of the sun, earth, jupiter...etc any or all of the various astronomical objects in the milky way galaxy. Particularly interested in the orbital velocities of the planets
  7. S

    I What Milky Way stars could go supernova to < +3 at any time?

    (I use +3 as the cutoff as that seems to be the limit of easily visible stars.) Yes, I know that Betelgeuse could go up in a < -10 blaze of glory, but I wonder what other ones are out there. On a side note, how fast could the big observatories move to it to observe it? And how quickly would...
  8. FactChecker

    B Are the stars that we see in constellations within the Milky Way galaxy?

    Are the stars that we see in constellations within the Milky Way galaxy? Should I assume that they are close and within the Milky Way?
  9. A

    B Our Sun's position in the Milky Way Galaxy

    Is this sentence correct? Our Sun lies in one of the spiral arms of the Milky Way, about 2/3 of the way out from the center of the galaxy.
  10. A

    B How does the Milky Way galaxy move in the local Group?

    How does the Milky Way galaxy move in the local Group? Is there a circular motion around the center of the local Group like the sun moves around the center of the galaxy?
  11. D

    B Accelerating to a stop in the Milky Way Galaxy

    If the sun is traveling around the center of the Milky Way at a speed of 536,865 mph and we are tagging along with it, couldn’t a spacecraft travel to another solar system by stopping? Would it be possible currently to slow down from that speed to 0 mph? And lastly, why does physics make my...
  12. Barbequeman

    Consider three models of the Mass distribution in the Milky Way

    I attached a Jpeg with my attempted solution but I´m not sure if I´m on the right way... I hope for a correction in my calculations
  13. M

    I Collision between the Milky Way and Andromeda and Redshift

    Hi ! My question is the following: If there is a redshift in astronomical observations with other galaxies, as our Milky Way galaxy is approaching with the Andromeda galaxy, they must surely be in an accelerated approach due to the force of gravity between the two galaxies, and as our galaxy...
  14. bbbl67

    I Andromeda vs. Milky Way galaxies

    Has their been any explanations found for why the Andromeda galaxy is so much bigger in diameter than the MW, yet at the same time it's lighter? Is the Andromeda a type of Low-Surface Brightness (LSB) galaxy? Also maybe somewhat related, why is the Andromeda's central black hole so much...
  15. K

    I Which forces maintain the shape of the Milky Way?

    Gravity, inertia, conservation of momentum and angular momentum are some of the forces and conserved quantities that I could think of which will tend to maintain the shape of the Milky Way. How is the separation of the spiral arms maintained?
  16. Michael_L

    I How to find the distance from a star to the center of the Milky Way Galaxy?

    The distance of the sun to the Galactic Center is about 7.4–8.7 kiloparsecs. If I use the known data of stars in various stellar catalogues, how can I calculate (approximately) a distance of a given star from the Center? What kind of data I need to look for? can it be calculated from Right...
  17. K

    I Question about Galactic Rotation curves in the Milky Way galaxy

    The graph in Wikipedia, article Milky Way, section Galactic Rotation, shows the actual rotation speeds in blue and the calculated speeds due to observed mass in red. (The graph is to the right of the article.) At about 3 kpc the actual speed is about 205 km/s. To account for the decrease in...
  18. jedishrfu

    B New 3D Map of the Milky Way

    https://www.inverse.com/article/58175-study-shows-the-milky-way-is-warped
  19. C

    B Milky Way is "in the top percentile of all the galaxies that exist"

    Our Galaxy is far bigger, brighter, and more massive than most others: Knowable Magazine Check out the amazing video, which shows all the many--more than 50!--satellite galaxies of the Milky Way burst into existence as they are discovered, year after year.
  20. S

    I Would we know if the Milky Way were a quasar?

    AIUI, a quasar is simply a black hole at the core of a galaxy that shoots out an enormous amount of energy along the axis of circulation. Since we are far from that axis, we cannot look into the Milky Way's core from that vantage point.
  21. A

    Do you think we are alone in the Milky Way?

    So.. recent estimations for the Drake Equation (Maccone, 2012) suggest that there could be around 4,600 civilizations in our galaxy that are able to release detectable signals. I find this number a little bit excessive, so I plugged some of the values of our Solar System into the equation and I...
  22. newrd

    B Andromeda Collision VS Expansion

    Hi Folks, I've read that by the time mankind is ready for intergalactic travel, our nearest galactic neighbour will be too far to even travel to. But I've also read that Andromeda is on a collision course with the Milky Way. Which is correct? And if the universe is expanding and everything is...
  23. S

    B What are these bright spots in this zoomable image of the Milky Way?

    I just recently found this zoomable image of the milky way and I have been really curious about what the 2 really bright spots are. There is one in the bottom left corner and another near the bottom center. https://www.eso.org/public/usa/images/eso1242a/zoomable/
  24. Sasho Andonov

    I Collision of Milky Way and Andromeda....?

    I do not understand how (in few bilion years) Milky Way and Andromeda will collide? If the universe is expanding and these objects which are far away have bigger speeds than those who are closer, than Milky Way and Andromeda will just be far away every moment... (?) Could someone help? :-)
  25. Simon Peach

    B The age of the Milky Way galaxy

    It seem incredulous to me that the Milky Way was formed just 13.775 million years after the start of the universe. If this is correct was it in the form that it is today?
  26. Sorcerer

    I GR in Milky Way: Approximating an Inertial Ref Frame?

    For the purposes of distance measurement, time measurement of signals, and so on, can we approximate the distance across the Milky Way as an extended inertial reference frame? Or is that way out of bounds? (I mean obviously there are massive bodies that already prohibit that, I suspect, but say...
  27. T

    B Did Astronomers Really Think the Milky Way Was the Only Galaxy in 1900?

    I read that in 1900 astronomers thought that our own Milky Way Galaxy was the only galaxy in the whole universe. This puzzles me because I presume they knew about the pinwheel structure of our galaxy and there were telescopes back in 1900( Yerkes for instance)which were large enough that could...
  28. nmsurobert

    I Milky way arms and the position of the Earth

    I've been trying to understand why we see different milky way arms depending on where we are on Earth and what time of year it is. I've been googling and I can't find an answer. Can anyone explain this to me? If you have a picture handy that would really help. Thanks.
  29. Lukeblackhill

    B Is the Milky Way the same as 'The Galaxy'?

    Good Morning, I've got a doubt originated from a statement of Berkeley Physics Course, Vol.1 - Mechanics, pg. 107, where in a footnote we read, "The systems are called galaxies, that one which contains our own sun is known as the Galaxy. The Milky Way is part of our galaxy". I've always...
  30. Suyash Singh

    Exploring the Width of the Milky Way: A Forgotten Dimension

    I was wondering that we always see the diameter of milky way but no one talks about its width. Even on google it is not there. This means that if I go up instead of forward then I would be out of milky in an instant?
  31. C

    I How Bright Would the Milky Way Look If It Had No Dust?

    Interstellar dust prevents us from seeing most of the light of the Milky Way. Does anyone know how bright it would look if it had no dust at all? As it is, the Milky Way is impossible to see from cities and many suburbs, but how would the situation improve if there were no dust between the stars?
  32. S

    I Actual number of stars in the Milky Way

    I hear wild varying estimates of how many stars there are in the Milky Way Galaxy, from 200 billion to 400 billion. Is the number closer to 200 billion or closer to 400 billion? What methods are being employed (other than GAIA) in order to count them?
  33. D

    Is the Parenago Discontinuity a real thing?

    Recently I came across a discussion on a Pantheist forum concerning the movement of cooler Stars & the Parenago Discontinuity. The proposal was that this disconinuity could be used as a way of testing whether cooler Star have self determination... yeah yeah I know, I'm not asking here for a...
  34. S

    I Do stars have any lower mass limit?

    Just read an article about a discovery of the smallest/least massive star in the Milky Way galaxy. The star has 85 times the mass of Jupiter and is known as EBLM J0555-57Ab located about 600 light-years from Earth. The entire article here -...
  35. V

    B Inclinations of star orbits in the Milky Way

    Do all stars orbit around the black hole in the same plane? Or some stars can have the same orbital radius but slightly different orbital inclinations?
  36. M

    I Solar system's path in the Milky Way galaxy

    My understanding: Circuit is ~ 30 million years. Plane of solar system orbit is tilted from plane of milky way. Is milky way plane crossed once or twice? What is error bar for 30 million? How much does dark matter density vary along solar system path?
  37. T

    B Did we just discover the Milky Way is in middle of nowhere?

    Did we really just discover the Milky Way is in middle of nowhere? I just saw this article today and I had never heard this before. In fact I thought it was a homogenous Universe apart from the cmbr cold spot and the dark flow area (apologies if they are the same thing)...
  38. TreeLover

    B An exercise related to the mass of the Milky Way, sort of.

    So, in preparation to the Portuguese Astronomy Olympiads, I've stumbled upon this problem (exercise): The sun, which is 8 kpc away from the centre of the Milky Way, has a rotation speed of approximately 220 kms-1 . Whereas a a star that is 15 kpc from the centre of the Galaxy orbits at a speed...
  39. Goatman

    B The Center of the Milky Way Galaxy?

    I am unsure on what exactly is at the center of the galaxy. I looked up some articles but some stated that scientists are unsure.
  40. Shellsunde

    I Dipole repeller - what is it?

    Recently, I read a few popularized articles portraying a study and associated calculations using astronomical measurements that posits a characterization of the direction and acceleration (or velocity) of our galaxy. The researchers assert not only is our galaxy being attracted by massive...
  41. L

    B Thickness & Distance of Milky Way at Sun's Location

    What is the thickness of the Milky Way at the sun's location, and how far is the sun toward the top/bottom?
  42. L

    B Retrograde star orbits in the Milky Way

    Are there any stars in the solar neighborhood that orbit the galactic center in the opposite direction?
  43. P

    B Are all galaxies moving away from the Milky Way?

    Once again I am reading and trying to understand more about the different theories of the beginning of time/space. I don't know why I never thought about this, but they say that galaxies are moving further and further away. One day we will not be able to see them because they moved so far. My...
  44. S

    Milky Way in Washington DC area

    I live in the Virginia suburbs of Washington DC where the light pollution is such that it is impossible to ever see the Milky Way in the sky. Where is the closest place one could go to actually see the Milky Way and more stars?
  45. J

    B Why the Milky Way shows band in all-sky energy scans

    I have just read about the Gleamoscope which allows users to dial up visions of the sky in any light that you prefer, for example CMB, Xray, etc. In all of these, the Milky Way shows up as a narrow horizontal band across the middle of theall-sky image. What is it in the analysis that leads to...
  46. J

    I How can Andromeda collide with the Milky Way?

    If everything in the universe is moving away from everything else in the universe, how can Andromeda collide in the distant future with the Milky Way?
  47. F

    I Orientation of the Earth, Sun and Solar System in the Milky Way

    I've been tinkering with a few diagrams in an attempt to illustrate the motion of the solar system in its journey around the Milky Way. I also wanted portray how the celestial, ecliptic and galactic coordinate systems are related to each other in a single picture. Note: in the Celestial, or...
  48. C

    B Found: The Least Luminous Galaxy Ever Seen

    Lying 280,000 light-years from Earth, the Virgo I galaxy emits only 180 times as much light as the Sun, half as much as the previous faint-galaxy champ. The galaxy's proximity to the Milky Way suggests that our Galaxy has hundreds of other satellite galaxies. New Scientist has the exclusive...
  49. Borg

    Stargazing GAIA - 1 billion stars and counting

    The GAIA telescope has been mapping stars in the Milky Way with unprecedented quality and quantities. It has been assembling the most detailed 3D map ever made of our Milky Way galaxy and has currently mapped over 1 billion stars. There are already hints that the Milky Way may be shaped...
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