Space

Are there more stars in the universe than all the sands on the Earth’s shores?

Published

4 weeks ago

25/08/2024

pouyan

Figuring out whether there are more stars in the universe than grains of sand on Earth’s beaches requires math and imagination.

Are there more stars in the universe than all the sands on the Earth’s shores?

One of the beloved aphorisms of the world of astronomy, coined by the late astronomer Carl Sagan, is that there are more stars in the universe than all the grains of sand on all the beaches on Earth.

It is hard to understand such a thing. When you stand on the beach, you can see countless sands. Considering all the beaches on Earth, the number of sand on the planet will be multiplied in countless ways. However, according to the famous saying, the number of stars in the universe is even more than the number of sands. It is really impossible to understand such a number.

Those who are familiar with the world of astronomy must have heard the mentioned sentence at least once. But Sagan’s aphorism, like so many incredible and nail-biting quotes, is worth asking a very basic question. Is it really true?

Unfortunately, it is not possible to give a direct one-word answer to the above question; Because finding the answer depends on many assumptions that must be considered, and determining some of them is not easy at all.

Finding the answer depends on many assumptions that must be considered and determining some of them is not easy at all

Very large and at the same time quantifiable problems that can be answered with approximate thumb calculations are called ” Fermi problems” in honor of Enrico Fermi, the famous physicist. Fermi was famous for finding ways to estimate large numbers in nearly intractable problems with enough accuracy to arrive at an approximate range of the correct answer.

Usually, when astrologers estimate things in general, they prefer to be accurate to a factor of 10; That is, the number they get is between one-tenth and 10 times bigger than the real answer. This is what astronomers call ” order of magnitude “. For example, one should not worry about an estimate that is two or three times less or more; Because it’s close enough to the real answer.

As a result, considering the dumbness of the answer we need, we return to the problem of sands and stars. First, let’s look at astronomy. For example, our Milky Way is a large galaxy consisting of hundreds of billions of stars. In fact, it is very difficult to determine this number; Because we are inside the galaxy and our view of much of it is blocked by obscuring gas and dust. It should be kept in mind that stars can have a wide range of luminosity. We can call the number 200 billion a conservative estimate.

Now we just need to multiply 200 billion by the number of galaxies in the observable universe to find out how many stars there are in the universe. A team of astronomers working on this issue published their results in 2016, stating that there are approximately two trillion galaxies in the universe.

So can we multiply 200 billion by two trillion to get the answer? Unfortunately, the issue is not that simple. Astronomers who estimated the number of two trillion actually took into account galaxies whose total mass of stars was more than a million times the mass of the Sun.

The estimate of one million is 200 thousand times less compared to the mass of the Milky Way! Consequently, we cannot use only the Milky Way as our model. The good news here is that low-mass galaxies, like stars, are likely to be much more abundant than more massive galaxies, and as a result, their higher numbers compensate for their lower star populations. Using a million solar masses per galaxy is probably close enough.

But there is another problem. A million solar masses do not mean a million stars in each galaxy! The Sun is unusually massive compared to most stars, most of which are actually smaller red dwarfs. Stars with the same or more massive mass than the Sun make up only about 10% of all stars; As a result, there are nearly 10 stars in the universe for every solar mass. We have to multiply 1 million solar masses of each galaxy by 10. Thus, an average of 10 million stars per galaxy is obtained.

So we can estimate the total number of stars as 10 million times 2 trillion = 20 million trillion = 20 quintillion, or 2 times 10 to the power of 19 stars. The universe does not lack stars at all.

But is the number obtained from the number of sands on the ground less or more? Now it’s time to go to the more down-to-earth estimates of the problem. The easiest way to estimate the number of sand grains on all the beaches of the world is to determine the volume of sand on those beaches, for example in cubic meters, and then multiply it by the number of sand grains per cubic meter. These numbers are not that hard to find.

How much sand is there in each cubic meter? The answer depends on the size of the sand grain, which ranges from less than 0.1 mm to almost 2 mm. Let’s take it as an average of one millimeter. Then one cubic meter will contain 1000 times 1000 times 1000 = one billion grains of sand.

Such a number is very high for just one cubic meter. Just a few hundred cubic meters of sand (about the size of an average house) is enough to contain all the stars in the Milky Way. As a result, we do not need a large amount of beach sand to match a large galaxy like our cosmic neighborhood.

From here on the numbers get more ambiguous. For example, how big is a beach? Suppose the size of the beach that is visible from the edge of the ocean to the higher ground reaches 50 meters and has a depth of 10 meters. Now we just need the length of all the beaches together.

Surprisingly, this number has been calculated: the total length of the coastline around all the continents of the Earth is approximately 2.5 million kilometers. Not all these beaches are sandy; But fortunately, the size of sandy beaches is determined: nearly 30% of the world’s coastline is sandy. We can be strict and remove the ant from the prime to arrive at a closer answer. In this case, we have 750 thousand kilometers or 750 million meters of sandy beach.

To find the total volume of sand, we must calculate as follows: 50 meters width multiplied by 10 meters depth multiplied by 750 million meters length = 375 billion cubic meters. If we go with the same estimate of one billion grains of sand per cubic meter, it means that there are 375 billion multiplied by one billion = 375 quintillion grains of sand. If we round the number to 400, then 4 times 10 to the power of 20 grains of sand can be found in the whole earth.

The answer obtained is actually 20 times the number of stars in the observable universe. As a result, the sentence of the aphorism of Sign is apparently wrong.

If we look at the bottom of the oceans and deserts, the amount of sand rises beyond imagination.

However, the assumptions of the problem were very superficial and this could greatly change the results. Consider the size of the sand grains: grains are on the smaller end of the spectrum, 0.1 mm, and can be a millimeter larger than the larger grains. If so, then there are one trillion grains of sand in every cubic meter, so the total number must be multiplied by a thousand. Even if we estimate the length and depth of the sand on a beach several times less or more, the sand wins over the stars by a factor of several thousand.

On the other hand, 10 million stars per galaxy may be too few. It is possible that most galaxies have many more stars than this. However, it is likely that the stars still cannot compensate for the huge advantage of the sands of the Earth’s beaches.

It should be kept in mind that we only considered the sands of the beaches. If we look at the bottom of the oceans and deserts, the amount of sand rises beyond imagination. The Great African Sahara alone probably has hundreds of times more sand than all the coasts of the earth.

Were you surprised by the result? This is the beauty of formal issues. You can quickly get estimated numbers and see how far they are from what you imagined. Our brains are not evolved to understand such huge numbers; As a result, it is not surprising that our imaginations are far from reality. But this is what mathematics and science are for; To prevent us from deceiving ourselves.

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Space

Why is Jupiter not a star due to its large size?

Published

13 hours ago

20/09/2024

pouyan

The smallest known star is smaller than Jupiter. So it can be said that Jupiter could also become a star? In this article, we answer this question.

Why is Jupiter not a star due to its large size?

The smallest known main-sequence star in the Milky Way is a red dwarf called EBLM J0555-57Ab, located 600 light-years from Earth. With an average radius of nearly 59,000 kilometers, this star is only slightly larger than the planet Saturn. Therefore, this red dwarf is the smallest known star that has hydrogen fusion in its core; The process that provides the star’s energy to burn until the end of its life.

In the solar system, there are two objects bigger than the mentioned star. One of them is the sun; But the other is the planet Jupiter , whose radius reaches 69,911 kilometers; But why is Jupiter a planet and not a star according to these dimensions?

The answer to the above question is simple: Jupiter does not have enough mass to support the hydrogen-to-helium fusion process. The star EBLM J0555-57Ab is nearly 85 times more massive than Jupiter. If this star was a little lighter, it would not be able to perform the hydrogen fusion process; But if the solar system had a different structure, would it be possible for the planet Jupiter to shine as a star?

Jupiter and the Sun are more similar than you might think

Jupiter may not be a star, but it has a huge influence on the solar system. The mass of this gas giant is 2.5 times the total mass of other planets in the solar system. On the other hand, Jupiter has a low density of 1.33 grams per cubic centimeter. While the density of the Earth is close to 5.51 grams per cubic centimeter, which is four times more than the density of Jupiter.

But it is interesting to point out the similarities between Jupiter and the Sun. The density of the sun is 1.41 grams per cubic centimeter. These two crimes are also very similar in composition. In terms of mass, nearly 71% of the sun is made up of hydrogen and 21% of it is made up of helium, and traces of other elements can be seen in it. On the other hand, 73% of Jupiter is made of hydrogen and 24% of it is made of helium.

Illustration of the planet Jupiter and its moon Io

For the above reasons, Jupiter is sometimes called a failed star; But again, Jupiter is unlikely to even come close to being a star. Stars and planets form in two completely different mechanisms. Stars form when a dense knot of matter in an interstellar molecular cloud collapses under its own gravity. This material begins to rotate in a process called cloud collapse. As rotation continues, more material from the surrounding cloud enters the stellar accretion disk.

With the increase in mass and as a result of gravity, the core of the baby star becomes more and more compact, which causes the temperature to increase and make it hotter. Finally, this mass becomes so compressed and hot that its core ignites and the process of thermonuclear fusion begins.

Based on our understanding of the star formation process, when a star runs out of accretion material, a full portion of its accretion disk remains. Planets form from this residue. According to astronomers, for gas giants like Jupiter, this process, called accretion, begins with small clumps of icy rocks and dust in the disk. With the rotation of these materials around the baby star, their density starts gradually and they stick to each other with the force of static electricity. Finally, these growing masses reach the size of nearly 10 times the mass of the earth; So that they can gravitationally absorb more gases from the surrounding disk.

From this stage, the gradual growth of the customer and its current mass began. The current mass of Jupiter is 318 times the mass of the Earth and 0.001 times the mass of the Sun. When a gas giant absorbs all its available matter, its growth stops. As a result, Jupiter has never even approached the mass of a star. The reason why Jupiter’s composition is similar to the Sun is not that it is a failed star; Rather, the reason for being born in the molecular gas cloud is the same as the sun.

Real failed stars

There are different groups of objects that can be classified as failed stars. These objects are called brown dwarfs and can fill the gap between gas giants and stars. The mass of brown dwarfs starts at 13 times the mass of Jupiter. These objects are heavy enough to support nuclear fusion, but this fusion is not of ordinary hydrogen but of deuterium or heavy hydrogen. Deuterium is an isotope of hydrogen that has one proton and one neutron in its nucleus instead of just one proton. The temperature and pressure of deuterium fusion is lower than the temperature and pressure of hydrogen fusion.

Since deuterium fusion occurs at lower mass, temperature and pressure, it is one of the steps to reach hydrogen fusion for stars whose accretion process continues and absorb the surrounding mass; But some objects never reach the required mass for hydrogen fusion.

Shortly after the discovery of brown dwarfs in 1995, these objects were called failed stars or ambitious planets, but numerous studies show that the formation of these objects like stars was from cloud collapse, not core accumulation; Some brown dwarfs do not even have enough mass to fuse deuterium, making them difficult to distinguish from planets.

Jupiter has exactly the lower mass limit for cloud collapse; The minimum mass required for cloud collapse is approximately equal to the mass of the planet Jupiter. As a result, if the planet Jupiter was formed from the collapse of a cloud, we could place it in the group of failed stars; But data from NASA’s Juno probe suggests that Jupiter at least once had a solid core, which is more consistent with the theory of core formation.

Modeling shows that the upper limit of planetary mass and formation by core accretion method is less than 10 times the mass of Jupiter. As a result, the planet Jupiter is not included in the group of failed stars; But by thinking about the cause of this issue, we can get a better understanding of how the universe works. In addition, the planet Jupiter has a stormy, striped and twisted appearance, and the existence of humans would probably not be possible without this gas giant.

Space

Why doesn’t Jupiter have big and bright rings like Saturn?

Published

13 hours ago

20/09/2024

pouyan

Why doesn’t Jupiter have big and bright rings like Saturn? Despite Jupiter’s resemblance to its neighbor Saturn, this planet lacks bright and beautiful rings. What factors have contributed to the diminution of customer circles?

Why doesn’t Jupiter have big and bright rings like Saturn?

Considering the similarity of the planet Jupiter to its neighbor Saturn, it is natural to ask why this planet does not have clear and bright rings like Saturn. However, Jupiter has thin, narrow rings made up of dust that only shine when there is sunlight in the background. According to new research, these narrow rings lack brightness because the large Galilean moons prevent rocks and dust from accumulating around Jupiter. According to Stephen Kane, an astrophysicist at the University of California Riverside:

The fact that Jupiter doesn’t have brighter rings than Saturn has bothered me for a long time. If Jupiter had such rings, it would certainly appear brighter to us because this planet is much closer to Earth than Saturn.

Keen and his colleague Zhixing Li, an astrophysicist at Riverside University, ran a series of simulations of objects orbiting Jupiter to test the hypothesis of a giant ring system around Jupiter at some point in history. The aforementioned simulations considered the orbital motion of Jupiter and its four largest moons, known as the Galilean moons, which are: Ganymede (which is even larger than Mercury and is known as the largest moon in the solar system), Callisto, Io, and Europa. The researchers also included enough time for the formation of a ring system in their simulations. According to this modeling, Jupiter has not even had rings similar to Saturn in the past and is unlikely to have them in the future. Kane explains:

Giant and heavy planets have heavy moons and these moons prevent the formation of rings of matter. The Galilean moons of Jupiter, one of the largest in the Solar System, would quickly destroy any potential large rings that might be forming.

Jupiter has narrow rings, most of which are dust from moons and material that may have been thrown into space by impact events. On the other hand, much of Saturn’s rings are made up of ice, possibly fragments of comets, asteroids, or icy moons that have been broken apart by Saturn’s gravity.

We know that Saturn’s moons play a vital role in the formation and maintenance of its rings, But one or more large moons can also gravitationally disrupt the rings and drive the ice out of the planetary orbit and into an unknown region. Although most people think that Saturn is the only planet with rings, rings around planets are very common even in the solar system. For example, in addition to Jupiter, the solar system’s ice giants Uranus and Neptune both have narrow rings of gas and dust.

Compared to other planets, Uranus has a strong axial deviation and its orbital axis is parallel to the orbital plane. The position of the rings of this planet is adjusted accordingly. Probably, a mass collided with Uranus and led to its axial deviation, or possibly this planet once had huge rings that caused this deviation. Of course, rings are not limited to planets. A small body with a width of 230 km called Chariklo, which is located in the orbit between Jupiter and Uranus, also has rings.

Also, the dwarf planet Haumea in the Kuiper belt has a ring. Simulations show that rings around ice masses are common due to the gravitational interaction and removal of ice from these masses.

Mars is also likely to be ringed in the future. The moon of Mars, Phobos, comes a little closer to this planet every year. Over the next hundred million years, the moon will come close enough to Mars that the planet’s gravity will break it apart, forming a short-lived ring that may recondense into a moon. Even Saturn’s rings may be temporary and rain down on the planet in the future. If we can study the rings in great detail, we can use them to fit together the puzzle pieces of planetary history. Kane believes:

To us astronomers, the rings are like bloodstains on a crime scene wall. When we look at the rings of the giant planets, we find evidence of the events that caused this material to accumulate.

Anyway, now that Jupiter has no spectacular rings, let’s enjoy Saturn’s rings. The Planetary Science Journal has accepted this research and is available on the arXiv database.

Space

Why do none of the moons of the solar system have rings?

Published

1 day ago

19/09/2024

pouyan

Many different moons have been discovered in the solar system. However, they all have one thing in common: none of them have rings.

Why do none of the moons of the solar system have rings?

We have many strange moons in our solar system. hot and cold moons; Moons with liquids and dusty moons. One lunar planet is walnut-shaped and another is potato-shaped; But among almost 300 moons that have been discovered so far, not even one of them has rings. This is really strange.

Of the eight planets in the solar system, half have rings of dust and ice that orbit their equator. It is thought that Mars once had a ring, and according to new research, even our blue planet probably had a ring similar to Saturn’s ring about 500 million years ago, which lasted for tens of millions of years.

In addition, some dwarf planets also have rings; Although astronomers have not yet been able to understand how these rings are formed. Even some asteroids have their own rings.

While investigating the concept of ringed moons outside our solar system, Mario Socercchia, an astrophysicist at the Universidad Adolfo Ibánez in Chile, and his colleagues became involved in the question of why moons in our own cosmic neighborhood lack rings. In an interview with Science Alert, he explains:

If the giant planets of the solar system have rings, and if the asteroids beyond the orbit of Jupiter and the non-Neptunian bodies also have rings, why don’t the moons of the solar system have rings? This absence is illogical considering the presence of rings in other places. As a result, it is better to investigate whether there are underlying dynamical reasons that prevent the formation of rings or their long-term stability around moons.

James Webb Space Telescope image of the rings of the planet Uranus.

We have yet to definitively discover an extrasolar moon, but in 2021 Soserkia and his colleagues hypothesized that if a moon had a large ring system, it could engineer its existence by blocking enough starlight. But the group later realized that we have yet to see any ringed moons, so the likelihood of their existence is very low.

When you’re an astronomer with a question in mind and a simulation tool at hand, there’s only one thing you can do: build models of cosmic systems and see what happens when you set them in motion.

There are many raw materials from which rings can form around the moons of the solar system. Some of these materials are dust resulting from the formation of impact craters. Some other moons emit steam or gas of their own, so there seems to be no problem with ring formation.

Considering the gravitational influence of the moon, host planet and other moons, researchers designed and tested physical N simulations and realized that due to these variables, ring formation around moons is difficult.

e ring of saturn — Saturn’s E ring formed from material ejected from the icy moon Enceladus

For example, Saturn’s moon Enceladus releases water vapor, ice particles, and gases from its glaciers in the Antarctic region with its remarkable surface activity. However, instead of forming a ring around this moon, these materials are transported into Saturn’s orbit by strong interactions with neighboring moons, feeding Saturn’s E ring.

In other words, even though the moons produce part of the raw materials necessary for the ring, their surrounding environment makes a large part of these materials available to the host planet and prevents the formation of the ring around the moons themselves.

So far, NASA has discovered 293 moons in the orbit of the planets of the solar system, most of which belong to the planets Jupiter and Saturn. Also, moons around dwarf planets and even asteroids have been discovered.

Soserkia and his team simulated the moons of a variety of solar system objects, from the Earth’s moon to the larger moons of Jupiter and Saturn, over millions of years of evolution. They sought to investigate the stability of these objects, their gravitational environment, possible ring systems, and their changes over time. The results of the investigation were contrary to the expectations of the researchers. Susarkia explains about this:

At first I expected rings to be completely unstable, which directly answered our question. However, contrary to expectation, we found that these structures have maintained their stability in many conditions. Indeed, in a previous paper we showed that isolated moons can have stable rings, but we did not predict that moons would remain stable in harsh gravitational environments despite the large number of other moons and planets that have distributed their rings. Another surprise came when we realized that these harsh environments, instead of destroying the rings, beautified them by creating structures like cracks and waves, which were just like what we see in Saturn’s rings.

Saturn’s moon Iapetus with its prominent equatorial ridge.

Some features of the moons of the solar system are signs of the past of the rings. The simulations suggest that the pebbles found orbiting Saturn’s moon Rhea could be the last remnants of a complete ring system. Also, Saturn’s moon Iaptus has a equatorial groove, which could be the remnant of a ring that fell on this moon, and in this sense, it is just like Saturn’s rings that slowly fall on this gas giant.

The findings show that the reason we do not see rings in the solar system today is that we are not in the right time and place. Solar radiation pressure, magnetic fields, internal heating, and magnetospheric plasma all contribute to the loss of once-existing lunar rings. According to Susarkia:

I believe we are unlucky to some extent; Because we started observing the universe during a period when these structures no longer exist. After doing this research, I was convinced that these rings probably existed in the past.

On the other hand, the only reason we see Saturn’s rings is because we are in the right place and time. For this reason, we see solar and lunar eclipses; Because the moon is gradually moving away from the earth and at some point it will be so far that it can no longer completely cover the sun.

The glory of Saturn’s rings.

The researchers believe that further simulations that take into account more parameters, such as beam pressure and magnetic fields, can help us understand the absence of lunar rings in more detail. We should also look more closely at the moons and look for evidence of the past, such as the craters on Iaptus.

At the same time, Suserkia and his colleagues are looking to expand their search and look for moons of rings around alien extrasolar worlds. He explains:

I wonder what mythical and epic stories we will hear from the inhabitants of other worlds about ringed moons. I mean, how will their stories and culture about the moons of the rings be different from our stories? There are infinite possibilities.

The scientists’ research has been accepted for publication in the Journal of Astronomy and Astrophysics and is available in the archive database.