Question

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what is galaxy? How many galaxies are there in universe?​

Answer 1

Answer:

Ordinarily, we point telescopes at some object we want to see in greater detail. In the 1990s astronomers did the opposite. They pointed the most powerful telescope in history, the Hubble Space Telescope, at a dark patch of sky devoid of known stars, gas, or galaxies. But in that sliver of nothingness, Hubble revealed a breathtaking sight: The void was brimming with galaxies.

Astronomers have long wondered how many galaxies there are in the universe, but until Hubble, the galaxies we could observe were far outnumbered by fainter galaxies hidden by distance and time. The Hubble Deep Field series (scientists made two more such observations) offered a kind of core sample of the universe going back nearly to the Big Bang. This allowed astronomers to finally estimate the galactic population to be at least around 200 billion.

Why “at least”? Because even Hubble has its limits.

The further out (and back in time) you go, galaxies get harder to see. One cause of this is the pure distance the light must travel. A second reason is due to the expansion of the universe. The wavelength of the light of very distant objects is stretched (redshifted), so these objects can no longer be seen in the primarily ultraviolet and visible portions of the spectrum Hubble was designed to detect. Finally, theory suggests early galaxies were smaller and fainter to begin with and only later merged to form the colossal structures we see today. Scientists are confident these galaxies exist. We just don’t know how many there are.

In 2016, a study published in The Astrophysical Journal by a team led by the University of Nottingham’s Christopher Conselice used a mathematical model of the early universe to estimate how many of those as-yet-unseen galaxies are lurking just beyond Hubble’s sight. Added to existing Hubble observations, their results suggested such galaxies make up 90 percent of the total, leading to a new estimate—that there may be up to two trillion galaxies in the universe.

Such estimates, however, are a moving target. As more observations roll in, scientists can get a better handle on the variables at play and increase the accuracy of their estimates.

Which brings us to the most recent addition to the story.

After buzzing by Pluto and the bizarre Kuiper Belt object, Arrokoth, NASA’s New Horizons spacecraft is at the edge of the solar system cruising toward interstellar space—and recently, it pulled a Hubble. In a study presented this week at the American Astronomical Society and soon to be published in The Astrophysical Journal, a team led by astronomers Marc Postman and Tod Lauer described what they found after training the New Horizons telescope on seven slivers of empty space to try and measure the level of ambient light in the universe.

Answer 2

Answer:

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Explanation:

Galaxies are sprawling systems of dust, gas, dark matter, and anywhere from a million to a trillion stars that are held together by gravity. Nearly all large galaxies are thought to also contain supermassive black holes at their centers. In our own galaxy, the Milky Way, the sun is just one of about 100 to 400 billion starsthat spin around Sagittarius A*, a supermassive black hole that contains as much mass as four million suns.

The deeper we look into the cosmos, the more galaxies we see. One 2016 study estimated that the observable universe contains two trillion—or two million million—galaxies. Some of those distant systems are similar to our own Milky Way galaxy, while others are quite different.

Types of galaxies

Before the 20th century, we didn't know that galaxies other than the Milky Way existed; earlier astronomers had classified them as as “nebulae,” since they looked like fuzzy clouds. But in the 1920s, astronomer Edwin Hubble showed that the Andromeda “nebula” was a galaxy in its own right. Since it is so far from us, it takes light from Andromeda more than 2.5 million years to bridge the gap. Despite the immense distance, Andromeda is the closest large galaxy to our Milky Way, and it's bright enough in the night sky that it's visible to the naked eye in the Northern Hemisphere.

In 1936, Hubble debuted a way to classify galaxies, grouping them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.