Galaxies
90 questions tagged Galaxies — spanning every one of the seven Ws.
Why do some galaxies have spiral arms?
Spiral arms form because waves of higher density move through a galaxy’s disk, compressing gas and triggering new star formation. The arms are patterns, not rigid structures, and they can persist for hundreds of millions of years.
Why do elliptical galaxies look more reddish than spiral galaxies?
Elliptical galaxies are usually redder because they contain older stars and little cold gas for making new blue stars. Without much fresh star formation, their light is dominated by aging stellar populations.
When was the first redshift survey of galaxies published?
One of the earliest major galaxy redshift surveys was published in 1977. It helped map the large-scale structure of the universe.
When was the first Hubble Deep Field image released?
The first Hubble Deep Field image was released in 1996. It revealed thousands of galaxies in a tiny patch of sky.
When was the first galaxy discovered beyond the Milky Way?
Edwin Hubble showed in 1924 that the Andromeda “nebula” was outside the Milky Way. This established it as a separate galaxy.
When was the first quasar discovered?
The first quasar was identified in 1963. It was 3C 273, one of the brightest and most famous quasars.
Where is the nearest large galaxy to the Milky Way?
The nearest large galaxy is Andromeda, in the constellation Andromeda. It lies about 2.5 million light-years away.
Which galaxy is the largest known spiral galaxy?
The largest known spiral galaxy is Malin 1, a giant low-surface-brightness galaxy about 1 billion light-years away. Its disk spans roughly 650,000 light-years across.
Which galaxy is the most massive known in the nearby universe?
The most massive known nearby galaxy is IC 1101, a giant elliptical galaxy at the center of the Abell 2029 cluster. It is often estimated to contain several trillion stars.
Which galaxy is the nearest large spiral galaxy to the Milky Way?
The nearest large spiral galaxy to the Milky Way is Andromeda, also known as M31. It is about 2.5 million light-years away.
Which galaxy has the highest known star-formation rate?
One of the highest known star-formation rates has been observed in starburst galaxies such as the Cosmic Eyelash, a strongly lensed galaxy forming stars extremely rapidly in the early universe. Its rate is hundreds of solar masses per year, far above the Milky Way’s roughly 1 to 2.
Which known galaxy is the brightest in radio wavelengths?
One of the brightest radio galaxies known is Cygnus A. It is a powerful active galaxy whose jets and lobes make it a landmark object in radio astronomy.
Which galaxy is the most distant well-studied spiral galaxy candidate?
Some of the most distant spiral galaxy candidates observed by modern telescopes are from the early universe and are still being refined by new data. A notable recent example is a surprisingly mature spiral structure seen in distant galaxies less than 2 billion years after the Big Bang.
Whose observations first measured the distance to another galaxy?
Edwin Hubble used observations of Cepheid variable stars to measure the Andromeda Nebula’s distance in 1923. This showed it was a separate galaxy.
How do scientists measure the mass of a galaxy?
They look at how fast stars, gas, or satellite galaxies orbit the galaxy and apply gravity. The result usually shows far more mass than the visible stars alone can explain.
How do astronomers know a galaxy is in a cluster?
They measure the galaxy’s position and velocity and check whether it is gravitationally bound to nearby galaxies. Cluster members often share similar redshifts and a common environment.
How does a supermassive black hole affect the galaxy around it?
It can regulate star formation by heating or expelling gas, especially when it is actively feeding. The black hole’s gravity also influences stars close to the galactic center.
How do galaxies merge?
Gravity pulls galaxies together over millions to billions of years until they interact, distort, and eventually combine. Their stars usually do not collide directly because space between stars is so large.
How does a quasar shine so brightly?
A quasar is powered by gas falling into a supermassive black hole at the center of a young galaxy. The infalling material heats up and radiates enormous energy before crossing the event horizon.
How do galaxies get their spiral arms?
Spiral arms are patterns that move through a galaxy’s disk, compressing gas and triggering star formation. They are not fixed lines made of the same stars forever.
Why does the Milky Way have spiral arms instead of being a smooth disk?
Spiral arms are not fixed structures of stars; they are density waves moving through the galaxy. Gas compresses in those regions, triggering new star formation and making the arms stand out.
Why do spiral galaxies keep forming new stars in their arms?
Spiral arms compress gas and dust, making it easier for clouds to collapse under gravity. That leads to bursts of star formation, especially in bright young clusters.
Why do some galaxies look blue while others look red?
Blue galaxies are usually forming many young, hot stars, while red galaxies contain older stellar populations with little new star formation. Dust can also make galaxies appear redder.
Why do elliptical galaxies usually have fewer new stars than spiral galaxies?
Elliptical galaxies have little cold gas left, and without that gas there is little material to make new stars. Many of them are dominated by old stars that formed long ago.
Why do galaxies collide without their stars usually hitting each other?
Stars are so far apart that even in a galaxy collision, direct star-to-star collisions are extremely unlikely. Instead, gravity distorts the galaxies, and gas clouds collide and form new stars.
What is a quasars?
A quasar is an extremely bright active galactic nucleus powered by a supermassive black hole. Its light can outshine the entire galaxy around it.
What is a Seyfert galaxy?
A Seyfert galaxy is a spiral galaxy with a bright active nucleus. It is a less extreme type of active galactic nucleus than a quasar.
What is a barred spiral galaxy?
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure of stars. The Milky Way is one example.
What is an irregular galaxy?
An irregular galaxy is a galaxy without a clearly defined spiral or elliptical shape. Many look distorted because of gravitational interactions or collisions.
What is a galaxy merger?
A galaxy merger is when two or more galaxies collide and combine under gravity. These events can trigger bursts of star formation and reshape the galaxies involved.
What is a tidal tail in a galaxy interaction?
A tidal tail is a long stream of stars and gas pulled out of a galaxy by gravitational interaction. It often forms during a close encounter or merger.
When was the first confirmed supermassive black hole discovered in a galaxy?
In the 1960s, observations of active galaxies like M87 and quasars revealed evidence for supermassive black holes.
When was the first evidence for galaxies beyond the Milky Way established?
In 1924, Edwin Hubble showed that the Andromeda Nebula was a separate galaxy beyond the Milky Way.
Where is the center of the Milky Way galaxy observed from Earth?
From Earth, the center of the Milky Way is seen in the direction of the constellation Sagittarius. It lies about 26,000 light-years away.
Where is the Large Magellanic Cloud visible from Earth?
The Large Magellanic Cloud is visible in the southern sky, mainly from the Southern Hemisphere, near the constellations Dorado and Mensa.
Where is the Small Magellanic Cloud found?
The Small Magellanic Cloud is a dwarf galaxy visible in the southern sky, near the constellation Tucana.
Where is the Triangulum Galaxy located in the night sky?
The Triangulum Galaxy lies in the constellation Triangulum and is about 2.7 million light-years from Earth.
Where is the Sombrero Galaxy located?
The Sombrero Galaxy is in the constellation Virgo, about 31 million light-years away.
Where is the Whirlpool Galaxy found?
The Whirlpool Galaxy is in the constellation Canes Venatici, about 23 million light-years away.
Where is the Pinwheel Galaxy located?
The Pinwheel Galaxy lies in the constellation Ursa Major, about 21 million light-years from Earth.
Which galaxy is the most massive in the local universe?
In the Local Group, the Milky Way is not the most massive galaxy; Andromeda is generally estimated to be larger and more massive. Its mass is commonly placed around 1.2 trillion solar masses, though estimates vary.
Which galaxy has the largest known star-forming ring?
The Cartwheel Galaxy is famous for its enormous ring-like structure and star formation triggered by a collision. Its outer ring spans about 150,000 light-years across.
Which galaxy is the brightest known radio galaxy?
Cygnus A is one of the brightest and most powerful radio galaxies known. It is an important nearby example of an active galaxy with giant radio jets.
Whose research first suggested that quasars are powered by supermassive black holes?
Martin Rees was among the key scientists who helped develop the black-hole model for quasars. His work in the 1960s and 1970s shaped modern ideas about active galactic nuclei.
Whose telescope data first revealed that the universe is expanding?
Edwin Hubble’s. His galaxy observations in the 1920s showed recession velocities increasing with distance.
Whose work first showed that galaxies are outside the Milky Way?
Edwin Hubble’s. He identified Cepheid variables in Andromeda in 1923–24, proving it is a separate galaxy.
How do galaxies form and grow over time?
Small clumps of matter formed first, then gravity pulled gas together into stars and merged smaller systems into larger galaxies. This hierarchical growth continues today through mergers and gas accretion.
How do spiral galaxies get their arms?
Their arms are thought to be density waves, not fixed material structures, that compress gas and trigger star formation as stars and gas move through them. The arms can persist for long periods even though the stars inside them orbit the galaxy.
How do elliptical galaxies differ from spiral galaxies?
Elliptical galaxies are smoother and rounder, with little cold gas or ongoing star formation, while spiral galaxies have disks, arms, and more active star birth. Ellipticals are often made of older stars.
How does a galaxy become a radio galaxy?
A supermassive black hole at the center can launch powerful jets that accelerate particles and emit strong radio waves. These jets can stretch far beyond the visible galaxy.
How do astronomers know the Milky Way has spiral arms if we live inside it?
They map stars, gas, and dust using infrared observations, radio surveys, and measurements of stellar distances and motions. These data reveal the Galaxy’s spiral structure from the inside out.
How do astronomers know a galaxy has merged with another galaxy in the past?
They look for streams of stars, disturbed shapes, unusual star motions, and leftover gas or shells. These are the fossil traces of ancient collisions.
Why do stars near the center of the Milky Way move so fast?
They orbit a huge concentration of mass, including the supermassive black hole Sagittarius A*. The closer a star is to the center, the faster gravity makes it move.
Why is the Milky Way a spiral galaxy instead of a smooth ball of stars?
The galaxy still has rotation, gas, and density waves that organize stars and star-forming regions into spiral arms. Those arms are patterns, not rigid material structures.
Why are galactic collisions common even though stars are far apart?
Galaxies contain huge numbers of stars and move through space under gravity, so over billions of years their paths often cross. Individual stars rarely collide, but the galaxies themselves strongly interact and merge.
Why do some galaxies look red while others look blue?
Blue galaxies usually have lots of young, hot stars and active star formation, while red galaxies are dominated by older stars with little new star birth. Dust can also make galaxies look redder.
Why does the universe contain so many galaxies of different sizes?
Galaxies grow by collapsing gas and by merging with smaller galaxies over billions of years. Different environments and histories produce dwarfs, spirals, ellipticals, and irregulars.
Why do galaxy clusters contain hot X-ray gas?
As galaxies fall into a cluster’s deep gravity well, gas is heated to tens of millions of degrees. That superheated gas glows strongly in X-rays.
Why do some galaxies have supermassive black holes at their centers?
Gas inflows, mergers, and repeated feeding over cosmic time can build black holes millions or billions of times the Sun’s mass. Many large galaxies, including the Milky Way, seem to host one.
Why do some galaxies appear to have no spiral arms at all?
Not all galaxies are rotating gas disks. Elliptical galaxies and many irregular galaxies lack the conditions needed to form clear spiral patterns.
What is a spiral arm in a galaxy?
A spiral arm is a curved region of a spiral galaxy where gas, dust, and young stars are concentrated. It is not a solid arm but a density pattern.
What is a dwarf galaxy?
A dwarf galaxy is a small galaxy with far fewer stars than a large spiral or elliptical galaxy. Some dwarf galaxies contain only a few million stars.
When was dark matter first proposed?
Dark matter was first proposed in 1933 by astronomer Fritz Zwicky.
When was the first galaxy cluster catalog published?
The first major galaxy cluster catalog is commonly traced to Abell’s catalog, published in 1958.
When was the first radio galaxy discovered?
The first radio galaxy, Cygnus A, was identified in the early 1950s.
When was the first observation of a star cluster in another galaxy made?
Extragalactic star clusters were recognized in the early 20th century, with key work accelerating in the 1920s and 1930s.
When was the first evidence for galaxies beyond the Milky Way published?
Edwin Hubble published decisive evidence for galaxies beyond the Milky Way in 1924.
Where is the Virgo Cluster of galaxies located in relation to the Milky Way?
It lies about 54 million light-years away, in the direction of the constellation Virgo. It is the nearest large cluster of galaxies to our own Local Group.
Where is the Small Magellanic Cloud located in the night sky?
It is in the southern sky near the constellations Tucana and Hydrus. Like the Large Magellanic Cloud, it is a satellite galaxy of the Milky Way and is visible to the naked eye from the Southern Hemisphere.
Where is the Whirlpool Galaxy located?
It is in the constellation Canes Venatici, about 23 million light-years away. It is interacting with a smaller companion galaxy, NGC 5195.
Where is the Triangulum Galaxy located?
It is in the constellation Triangulum, about 3 million light-years away. It is one of the nearest major galaxies and part of the Local Group.
Where is the Sculptor Galaxy located?
It is in the constellation Sculptor, about 11 million light-years away. It is one of the closest starburst galaxies to the Milky Way.
Where is the Centaurus A galaxy located?
It is in the constellation Centaurus, about 12 million light-years away. It is one of the nearest radio galaxies and active galactic nuclei to Earth.
Where is the Shapley Supercluster located?
It is in the direction of the constellation Centaurus, about 650 million light-years away. It is one of the most massive concentrations of galaxies in the nearby universe.
Which galaxy is the largest known by diameter?
IC 1101 is often cited as the largest known galaxy by diameter, with estimates around 1 to 2 million light-years across. It sits at the center of the Abell 2029 galaxy cluster.
Which galaxy is the nearest large galaxy to the Milky Way?
Andromeda is the nearest large galaxy to the Milky Way, at about 2.5 million light-years away. It is on a collision course with our galaxy over billions of years.
Which galaxy group is the nearest major galaxy cluster to the Local Group?
The Virgo Cluster is the nearest large galaxy cluster to our Local Group, about 54 million light-years away. It contains more than a thousand galaxies.
Which galaxy is the largest member of the Local Group?
The Andromeda Galaxy is the largest member of the Local Group. It contains around a trillion stars, more than the Milky Way’s estimated several hundred billion.
Whose observations first showed that galaxies come in many forms, not just “spiral nebulae”?
Edwin Hubble classified galaxies into ellipticals, spirals and irregulars in the 1920s. His work helped modernize galaxy astronomy.
Whose observations linked galaxy redshifts to cosmic expansion?
Edwin Hubble is best known for showing that galaxies recede faster when they are farther away. The relationship is now called Hubble’s law.
Whose work first explained why galaxies move the way they do in clusters and superclusters?
Vera Rubin’s galaxy rotation studies helped reveal that galaxies contain large amounts of unseen mass. That unseen component is now called dark matter.
Whose work first showed that galaxies come in many forms, not just “spiral nebulae”?
Edwin Hubble classified galaxies into spirals, ellipticals, and irregulars. His system made galaxies a major field of astronomy.
Whose discovery showed that the universe contains galaxies beyond the Milky Way?
Edwin Hubble proved in the 1920s that the Andromeda “nebula” lies far outside the Milky Way. This showed the universe is much larger than previously thought.
Whose observations first showed that the Andromeda galaxy is moving toward the Milky Way?
Vesto Slipher measured Andromeda’s radial velocity in the early 20th century. He found it was blueshifted, meaning it is approaching us.
Whose observations first showed that the universe contains galaxy clusters and large-scale structure?
Harlow Shapley and later astronomers mapped clusters and superclusters, revealing that galaxies are not scattered randomly. Their work helped uncover the cosmic web.
Whose observations first showed that the Milky Way is just one galaxy among many?
Edwin Hubble’s 1920s work on Andromeda and other nebulae showed that they are external galaxies. This proved the Milky Way is not the whole universe.
How do astronomers estimate the age of a galaxy?
They combine the ages of its stars, its chemical makeup, and how far its light has been stretched by expansion. For nearby galaxies, detailed star-by-star studies can give ages of about 10 to 13 billion years.
How do spiral arms in galaxies stay together?
They are not solid structures; they are regions of higher density that move through the disk. As stars and gas pass through them, new stars are often born there.
How do galaxies collide without most stars crashing into each other?
Because stars are so far apart, direct collisions are rare. The galaxies’ gas, dust, and gravity interact strongly and can reshape both galaxies over millions of years.
How do astronomers determine the shape of a galaxy’s halo?
They track stars, globular clusters, gas, and dark matter signals extending far beyond the visible disk. These measurements show the halo is much larger than the bright parts of the galaxy.