If you have read this column with a leg, it is possible that you have noticed a trend recently in which you try to answer some deceptively simple questions: What is a planet? What is a moon? What is a star? A recurring theme here is that it is difficult to define such terms, the definitions are, well, definitionWhile nature is rarely like that.
According to the trend and the subject, it is time to step out, a Cerdilla Passed.
What is a galaxy?
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The word itelf comes Galaktikós Kyklosor “dairy circle”, the old Greek term for the Milky Way, our local galaxy.
In modern times, thought, we use “galaxy” more generically to refer to any large system of stars, gas, dust and dark matter, all linked by its mutual gravity. That is not a bad definition, except for the small fact that not all galaxies have all these characteristics. There are galaxies without detectable gas and dust, and some that seem to have little or no dark matter. I guess the common thread is that they all have stars, but at the time we find problems because not all gravitational stars systems are classified as galaxies.
Very generally he speaks, the galaxies are tens or thousands to hundreds of thousands of light years and have millions of several billion stars. Our Milky Way, for example, has about 120,000 light years wide and contains somewhere between approximately 100 billion and 400 billion stars. It is difficult to obtain only a difficult number because the weak red dwarfs are the most common child of the stars, but they are so DIM, they fade to invisibility equally relatively close to our solar system.
Even so, we do know the general form of our galaxy, which is a gigantic album with a central bulk of major stars and savior that has about 20,000 light years wide. The structure of the disc is dominated by four main spiral arms and several smaller. These arms are splashed by huge clouds of gas and dust from which the stars are born. Many of these stars are massive, bright and blue, which gives spiral arms its characteristic blue tone. Because we are inside the galaxy, the mapping is spread, there are still many details of its structure.
The spirals are only one of the four main classifications for galaxies; The others are elliptical, irregular and peculiar.
Elliptic galaxies would be called better spheroidal; They can be spherical but they are more common, a bit like a cotton ball. Some of the biggest galaxies we know are elliptical, such as the powerful M87 galaxy, which has several billion stars. M87 is located in the center of the Virgo cluster, a large group of galaxies linked by gravity that has about 2,000 members. In fact, such giant elliptical are common in the centers of the galaxies groups. These galaxies grow by eating others, both large and small, which venture the grouping core too much, so the elliptical form can be partly falls out of this cannibedism process.
However, not all elliptics are huge: many are quite small. These are called “dwarf elliptical galaxies” and are larger galaxies satellites. The Andromeda Galaxy, for example, has many dwarf elliptical that orbit it.
The elliptics do not have much gas in them, if there is. They seem to have exhausted Essential and all their gas when making stars a long time ago. The big blue stars do not live much; They explode as supernovae within millions of years of their birth, so a few billions of years after their gas are exhausted, an elliptical galaxy will look very red, dominated by more rough stars of low dough.
Peculiar galaxies are rare. They have all kinds of ways, but they tend to be, well, strangers. They are the usual results of galactic collisions, trains wrecks at cosmic scale. The mutual gravity of two galaxies that interact is stretched and distorted its regular forms, creating long lines and hung star curves. Sometimes, if a narrower galaxy crosses the center of a larger one, the collision creates a ring galaxy, in which the stars and gas move away from the center of the largest galaxy as waves in a pound after a rock falls.
Finally, there are irregular ones. These tend to be at the small end of the galactic scale and, as their categorization suggests, they have no general shape. The small Magelian cloud (or SMC), a complementary satellite of the Milky Way, is an irregular galaxy. His brother, the great Magelian cloud (LMC), seems irregular to the beginning to the eyes, but now we know that it has some structure in the form of a single spiral arm. Both galaxies are small compared to the Milky Way; The SMC has a few hundred millions of stars and has just under 20,000 light years wide, while the LMC has billions of stars and has more than 30,000 light years wide.
The existence of irregular raises an important question: how small can a galaxy be? The current paradigm is that the galaxies initially formed shortly after the Big Bang. Huge clouds of invisible dark matter accumulated regular matter (in the form of gas) through gravity, which collapsed to form galaxies and the stars in them. A study published in April 2025 in the Astrophysical Diary Letters He showed that there is a minimal mass necessary for such a dark cloud of matter to collapse, and it is about 10 million times the mass of the sun. This can then be a lower limit in the smallest galaxy that can be formed.
But this lower end of the galactic scale is still quite murky, in part because the stars that serve as observable tracers of small galaxies can form equally narrower structures called groups of stars. A child, called open groups, can have thousands of stars in them. The groups of globular stars, on the other hand, can house dozens of millions of stars that add up to several million times the mass of the sun. Even the largest known globular group (approximately 150 light years of light more or less) remains narrower than a dwarf galaxy. We believe that many of the largest or largest groups, such as Omega Centauri, actually once wake up However, the same dwarf galaxies. Its shrunken state is multiple passes near a larger galaxy that strip them of stars, leaving a smaller and more compact object.
What do we call Omega Centauri, then? A huge globular group or a partial digested dwarf galaxy? In addition, some dwarf galaxies seem not to contain any dark matter, and others have very ferwer stars than expected, even below the theoretical lower limit, further confusing the problem. In the closest end, apparently, the border becomes irritatingly confusing. That is also recurring when we try to create rigid definitions for cosmic objects. Humans love Sharp Delineeds, but the universe does not have the obligation to obey them.
One last thing: in 2016, a study showed that the universe can contain as many as Two billion Separate galaxies. The cosmos clearly likes to do galaxies, and although I define them, there is a fixed number for us to study and learn.
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