The Jerusalem Report. Jerusalem Post Lite. March of the living. Kabbalat Shabbat. Shapers of Israel. Maariv Online. Maariv News. Tools and services. JPost Premium. Ulpan Online. JPost Newsletter. JPost News Ticker. Our Magazines. Learn Hebrew. RSS feed. 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 billion.
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. 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. 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. The deeper we look into the cosmos, the more galaxies we see. One 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.
Since it is so far from us, it takes light from Andromeda more than 2. 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 , Hubble debuted a way to classify galaxies, grouping them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.
More than two-thirds of all observed galaxies are spiral galaxies. A spiral galaxy has a flat, spinning disk with a central bulge surrounded by spiral arms. That spinning motion, at speeds of hundreds of kilometers a second, may cause matter in the disk to take on a distinctive spiral shape, like a cosmic pinwheel.
Our Milky Way, like other spiral galaxies, has a linear, starry bar at its center. Elliptical galaxies are shaped as their name suggests: They are generally round but can stretch longer along one axis than along the other, so much so that some take on a cigar-like appearance. The universe's largest-known galaxies—giant elliptical galaxies—can contain up to a trillion stars and span two million light-years across.
Elliptical galaxies may also be small, in which case they are called dwarf elliptical galaxies. Elliptical galaxies contain many older stars, but little dust and other interstellar matter. Their stars orbit the galactic center, like those in the disks of spiral galaxies, but they do so in more random directions. Few new stars are known to form in elliptical galaxies. They are common in galaxy clusters. Lenticular galaxies , such as the iconic Sombrero Galaxy , sit between elliptical and spiral galaxies.
Like elliptical galaxies, they have little dust and interstellar matter, and they seem to form more often in densely populated regions of space. Galaxies that are not spiral, lenticular, or elliptical are called irregular galaxies. Irregular galaxies—such as the Large and Small Magellanic Clouds that flank our Milky Way—appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by.
They are full of gas and dust, which makes them great nurseries for forming new stars. Some galaxies occur alone or in pairs, but they are more often parts of larger associations known as groups, clusters, and superclusters. Our Milky Way, for instance, is in the Local Group , a galaxy group about 10 million light-years across that also includes the Andromeda galaxy and its satellites.
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