Modern Physics


Stars are large spheres formed by plasma heated to thousands of degrees. Its shape is due to its gravity, which points towards the star’s core.

Stars are large spheres of plasma , held together by their own gravity . Stars emit light, heat and other types of radiation due to the nuclear fusion processes that take place inside them, releasing large amounts of energy.

How are stars created?

Stars are formed by the condensation of gases that coalesce by gravitational attraction. Large nebulae, for example, are “nurseries” of stars, since, in their interior, large molecular clouds give rise to new stars. When the gases responsible for star formation approach each other, their speed increases, driven by local gravity, as well as their density and temperature.

Over a period, which can take up to 10 million years, these protostars (stars in an early stage of formation) are compressed by their own gravities until the pressure and temperature in their core are sufficient for hydrogen atoms to fuse, producing helium nuclei. Stars that derive their energy from the fusion of hydrogen atoms are called main sequence stars, this type of star accounts for about 90% of all stars in the Universe.

From the moment stars become capable of thermonuclear fusion , their fuel is consumed, until the star evolves into its final stage of life. The possibilities are many: according to the star’s mass and radius, it is possible to estimate what its future will be like. These stellar quantities, such as the mass and radius of stars, are commonly measured as a function of solar mass (M☉) and solar radius (R☉).

What are stars made of?

Most stars, whose masses range from 0.5M☉ (half the mass of the Sun) to 2.5M☉, are composed of helium and hydrogen, the most abundant elements in the Universe. That’s because these stars don’t have gravity or temperatures high enough to fuse heavier elements.

When stars are very massive: between 5M☉ and 10M☉ – like supergiants, elements heavier than helium are formed inside. The final stage of life for these stars is a supernova , a huge explosion that launches all of their matter and energy into space, giving rise to other stars and planets.

Life and Death of the Stars

The “lifetime” of stars depends on their mass: how quickly they consume their fuel is what tells how long the star maintains its brightness, the Sun, for example, consumes less than 0.01% of energy. its mass annually, increasing its temperature and luminosity. It is estimated that since the Sun became a main-sequence star 4.6 billion years ago, its brightness has increased by more than 40%.

Main sequence stars, called dwarf stars, are the absolute majority of stars in the Universe, our Sun, for example, is a yellow dwarf, a “low temperature” main sequence star when compared to the most recent stars. hot, like blue dwarfs. Check out some of the evolutionary stages of stars according to their mass:

  • Very low-mass stars: These stars, whose masses are up to half the mass of the Sun, eventually cool down after consuming the hydrogen in their interior, theoretically becoming white dwarfs formed exclusively by helium, however, the estimated lifetime for these types of stars are larger than the Universe itself, so existing stars will still become white dwarfs.
  • Low- mass stars: In their final stages of life, stars up to 2.5M☉ begin to form carbon and oxygen atoms in their core. With the decrease in their masses and the consequent decrease in their gravitational field, these stars become giants. During their expansion, these stars shed their outer layers, forming planetary nebulae.
  • Intermediate-mass stars: These stars have an evolution similar to low-mass stars, after their expansion, they leave behind only their core, giving rise to dwarf stars.
  • Massive stars: After having fused all their hydrogen, these stars expand, becoming supergiant stars, in this period, they start fusing heavy elements until their gravity can’t withstand the force of nuclear reactions, when this happens, these stars explode, launching your content through space at lightning speeds

types of stars

There are different types of stars. This designation depends on two things: the spectral classification, which concerns the temperature of the star, and the size and mass of the star. The spectral classification is given in color. In ascending order of temperature, we have the red, orange, yellow, yellow-white, white, blue-white and blue stars. Check out the image below about the evolution of stars:

Main sequence stars usually follow the steps above.

This color definition concerns the peak frequency emitted by the star and that related to the blackbody emission temperature . As stars produce almost all frequencies of radiation simultaneously, to the human eye they all appear whitish to the naked eye.

Check out some of the most important types of stars that exist:

  • Blue stars : They are extremely hot stars, their surface temperature can reach 30,000 K, they are very “new” stars compared to other types of stars. Most of these stars were created less than 40 million years ago.
  • Yellow dwarfs : Like the Sun, these stars are very old, having been around for billions of years. The future of these stars is to become a red giant.
  • Red dwarfs : They are the most common stars, representing about 73% of the stars in the Universe. Its brightness is weak, they are little massive stars.
  • Blue giants : These are very massive stars with temperatures above 10,000 K, which can have up to 250 times the mass of the Sun.
  • Blue supergiants : They are rare, extremely hot and bright, and can have up to a thousand times the mass of the Sun.
  • White dwarfs : These stars are formed by the cores of other stars that have ejected their outer layers, these stars no longer produce nuclear fusions and commonly rotate around their axes with very high speeds.
  • Neutron Stars : These are stars that have been so compressed that all of their protons and electrons are ejected due to electrical repulsion. They are very small, between 5 and 15 km in radius and their temperatures exceed hundreds of thousands of degrees Celsius.

In some cases, supermassive stars, with masses greater than three solar masses, can collapse, giving rise to black holes. Black holes do not allow light to escape from inside them because of their enormous gravity.

However, around the black holes it is possible to observe the accretion disks: they are the gases of other stars that orbit them. When accelerated towards the event horizon, the region of black holes from which nothing escapes, the gases are heated, starting to emit different frequencies of electromagnetic waves .

What is the number of stars in the sky?

Despite sounding simple, this is an extremely difficult question to answer, simply because it is not possible to count such a large number directly. It is estimated, however, that there are at least 10 10 galaxies in the observable Universe, which may contain a few billion stars.

In our galaxy, the Milky Way , and also in our nearest neighbor, the Andromeda Galaxy, for example, there are at least 100 billion stars, therefore astronomers’ estimates indicate that there must be at least 10 21 stars in the whole Universe.

Despite the huge number of stars, a tiny part of them is visible from Earth with the naked eye. From here, without the aid of any optical instrument, it is only possible to see about 10,000 stars.

star names

Currently, there are about 330 official and proper names given to the stars. Check out some of these names, as well as some characteristics of each of these stars:

  • Canis Majoris: The star VY Canis Majoris (scientific nomenclature) is one of the largest known stars, this hypergiant has about 1420 solar radii.
  • Sirius: Sirius is a binary star, the brightest in the sky, located 8.6 light years from Earth.
  • Canopus : It is the second brightest star in the sky, it is at a distance of 310 light years from Earth.
  • Aldebaran: It is a red giant, the brightest in the constellation of Taurus, located 65 light-years from Earth.
  • Rigel : It is the brightest star in the constellation of Orion and the seventh brightest star in the sky.
  • Betelgeuse : It is the twelfth brightest star in the sky and the brightest star in the constellation Orion.
  • Antares : It is a supergiant star, with a radius greater than 822 solar radii, it is located 600 light-years from Earth.
  • Canopus : It is a red supergiant, the brightest star in the constellation Carina.

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