Dead stars

The DEAD Stars in the UNIVERSE explained

Aug. 24, 2021

The DEAD Stars in the Universe! Hi, it's Aryan here. In this article, I will tell you about "dead" stellar objects, that is all those celestial bodies residual of the last evolution phases of a star's life: namely planetary nebula, white dwarf, supernova remnants, neutron star, pulsar, magnetars, and black holes.

How are they formed?

To grasp the idea of the formation of these celestial bodies, however, we must first briefly describe how stars are formed and how they evolve. All stars are formed within the densest areas of vast molecular clouds present in the interstellar medium, essentially composed of hydrogen, with a quantity of helium of 23-28% and traces of heavier chemical elements. These nebulae are also known as HII regions because the hydrogen from which they are composed is ionized by the ultraviolet radiation emitted by the stars that form inside them (HII indicates hydrogen ionized once).If one of these molecular clouds is crossed by the shock wave triggered by the explosion of a supernova or is compressed by a collision between galaxies, it begins to manifest gravitational instability and begins to contract due to its own force of gravity, and inside, in the densest areas of  the nebula, a protostar begins to form, a celestial body that emits radiation precisely because of the gravitational contraction. At this point, the subsequent stellar evolution depends on the mass of the star. If the mass is less than 8% of the mass of the Sun, the protostar becomes a brown dwarf: a celestial body slightly larger than the planet Jupiter, in which the only two sources of energy are the gravitational contraction and the fusion reactions of lithium and deuterium (an isotope of hydrogen whose nucleus is composed of a proton and a neutron): the relatively low temperatures of the nucleus (at most a few thousand degrees) prevent the establishment of thermonuclear fusion reactions typical of the most massive stars.

Brown dwarfs have typical masses between 13 and 65 times the mass of the planet Jupiter and in this mass range the fusion of deuterium occurs; under the limit of 13 Jupiter masses we no longer speak of brown dwarfs but of gas giant planets, above 65 Jupiter masses the fusion of lithium can also occur. Due to their very small masses, brown dwarfs cool down over time and their brightness decreases. Due to their very low brightness, brown dwarfs are not visible to the naked eye. If the mass of the star is larger than 8% of the mass of the Sun, the protostar continues towards the main sequence phase: a phase of stability in which the energy source in the stars comes from thermonuclear reactions that melt hydrogen. During this phase, the stars are in a condition of hydrostatic equilibrium, in which the radiation pressure generated by the thermonuclear reactions (which tends to make the star expand) counterbalances the force of gravity (which tends to cause the star to contract), and in the stellar nuclei temperatures are of the order of tens of millions of degrees.

To know more about Brown dwarfs, click here!

Even in the type of thermonuclear reactions there is a difference depending on the mass of the star: in stars with a mass less than 1.5 times the solar mass, the fusion of hydrogen occurs through the proton-proton chain, while in those with mass greater than 1.5 times the solar mass, the carbon cycle prevails. The main sequence phase has a different duration, again due to the different masses of the stars: the most massive stars in fact consume hydrogen faster so they remain in the main sequence for a few tens of millions of years; examples of this type of star are the Pleiades. Stars like the Sun and smaller, since they consume hydrogen more slowly, instead remain in the main sequence even for billions of years! In the case of our Sun, the main sequence phase is believed to have a total duration of about 10 billion years (currently our star is more or less in the middle of this phase); but think, the red dwarfs, even smaller than the Sun, could remain in the main sequence for longer than the age of the universe, which is currently almost 14 billion years old! It's unbelievable, isn’t it?

To give an idea, think about filling two vehicles with 50 liters of gasoline: a large truck and a small car. Which of the two vehicles will consume gasoline first? Obviously, the truck, because being bigger and more massive than a small car, needs a bigger engine to be able to work. What happens when hydrogen runs out in the core of a star? The star exits the main sequence phase: since the radiation pressure generated by the combustion of hydrogen fails, the star is no longer in a condition of hydrostatic equilibrium: it will therefore tend to contract. The outermost layers of the star will then press on the innermost ones and on the core, exerting a certain pressure on them. This increase in pressure on the central regions of the star will increase the core temperature, up to values ​​of the order of 100 million degrees, so that new thermonuclear reactions will be triggered: this time, however, it will no longer be hydrogen to burn, but helium and this process will produce carbon and some oxygen. The effect is that the star will expand a lot becoming a red giant: in the case of the Sun, it is believed that it will become such in about 5 billion years: if the Sun of this very distant future was in the place of the current Sun, it's outer gaseous envelopes would come to lick the orbit of Venus! And even the Earth would most likely not fare well: what could happen in your opinion? After the red giant phase, the evolution of stars continues differently depending on their mass. 

In the case of less massive stars, therefore also of our Sun, the outermost layers of the red giant will slowly expand into space, a bit like a swelling soap bubble, and will form a planetary nebula. It is important to point out that planetary nebulae have nothing to do with planets: it seems that this denomination was first proposed, around 1780, by William Herschel, to whom these expanding gas bubbles, observed through his telescope, clearly reminded of planetary systems in the process of formation. Observed through the telescope, the planetary nebulae appear round in shape, and the appearance of some is somewhat reminiscent of a donut or a smoke ring. Examples of planetary nebulae are M 27, which can be observed with a 20 cm telescope in the direction of the constellation of Vulpecula, and M 57, in the direction of the constellation of Lyra. And what happens to the rest of the star, i.e. the central part?

A white dwarf is formed, a very dense and compact object: think that a white dwarf is about the same size as our planet, but contains a quantity of matter equal to at most 1.4 times the mass of the Sun! The value of 1.4 solar masses is known as the Chandrasekhar’s limit, in honor of the astrophysicist Subrahmanyan Chandrasekhar, who derived it: this value represents the upper limit of the mass of a white dwarf. The densities of white dwarfs are insane, more or less in the order of 1830000 g / cm3! In practice, if you take a cube with a size equal to 1 cm, it would weigh about 1830 kg, almost 2 tons! What would happen to you if this cube fell on your foot? Write it in the comments! Due to their low luminosity, white dwarfs are not observable through the naked eye, but through Newtonian telescopes with at least 20 cm aperture, at least the brightest ones can be seen. One of the most famous white dwarfs is Sirius B, companion of the more famous star Sirius, in the constellation of Dog Major, just over 8 light-years from Earth. What happens to the most massive stars instead? 

They also pass through the red giant phase, but, due to their greater mass, the combustion of helium in the nucleus occurs much faster: when the helium runs out, the radiation pressure triggered by the combustion of the helium, and the same violently collapses on itself due to the force of gravity. This causes a sudden increase in temperature in the core and further thermonuclear reactions during which the chemical elements heavier than carbon and oxygen are produced; the very rapid increase in temperature causes the outer envelope of the star to expand violently in space, in an explosive way, thus producing a supernova explosion,  and originating a supernova remnant. Some famous examples of supernova remnants are also observable through an amateur telescope: for example the Crab Nebula M 1 in the direction of the constellation of Taurus, about 6300 light-years from Earth; the supernova explosion that originated it was observed by the Chinese in 1054, who recorded it as a "host star". According to Chinese documents, it reached a peak of the magnitude of -6 (so to speak, four times brighter than the planet Venus!). Petroglyphs found in Navaho Canyon and White Mesa in Arizona and in Chaco Canyon National Park in New Mexico appear to be representations of the event by the Anasazi Indians. 

M 1 was observed by the French astronomer Charles Messier in 1758, while he was looking for Halley's comet on its first expected return. Another very famous supernova remnant is the spectacular Swan Veil complex, much appreciated by astrophotographers. What remains of the nucleus also becomes a very dense and compact object, not a white dwarf, but a neutron star. Within these objects, the matter is degenerate: that is, it is so compressed that electrons and protons combine together to originate neutrons, due to the weak interaction. The range of masses typical of these celestial bodies is between 1.4 and 3 solar masses, therefore higher than the Chandrasekhar’s limit, while the diameter is around 10 km. If you have already been amazed by the density of a white dwarf, the density of a neutron star of 3 solar masses is even more incredible: 1015 g / cm3! In other words, it is almost 800 million times greater than the previous one! It is definitely good that a cube of neutron star matter does not fall on your head.

There are several types of neutron stars,  the most important being the following: Pulsars: These are neutron stars that emit two beams of radio waves from the poles and rotate very quickly on themselves, a bit like a lighthouse at the port. The fastest pulsars even spin 1000 times on themselves in one second! The first pulsars were discovered by astronomer Jocelyn Bell in 1967. They are not visible to the unaided eye, but through their emission in the radio band: if you have any amateur radio friends you can ask them to let you listen to the sound of pulsars. Magnetar: they are similar to pulsars, but are characterized by a very strong magnetic field, from 10^13 to 10^15 gauss, billions of times more intense than the earth's magnetic field (which is instead around 1 gauss).  Because of these very intense magnetics, magnetars also emit soft gamma rays. What if the star has an initial mass greater than 3 solar masses? After the supernova explosion, its core does not become a neutron star but collapses into a black hole: a celestial body with a gravitational field so immense that not even light can escape. A black hole, to our eyes, is invisible precisely because it does not emit light. From a relativistic point of view, a black hole is a region of spacetime with a very large curvature.

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