Buzz
Looking up at the night sky, it's impossible not to wonder, “What lies out there?” The beauty of the stars sparks a natural curiosity about the cosmos. Perhaps on a distant planet, light years away, there's another life form gazing at our sun, seeing it as just a tiny dot in the sky, and pondering the secrets it holds.
Though we may never fully comprehend the vastness of cosmology, that doesn't stop us from exploring it. From the well-known to the speculative, this list highlights ten truly remarkable star types.
10. Hypergiant
Although hypergiants might seem a bit underwhelming compared to other stars on this list, I couldn't resist mentioning them due to their immense size. It's nearly impossible to truly grasp how gigantic these stars are, but the largest known star, NML Cygni, has a radius 1,650 times that of our sun—about 7.67 AU. For comparison, Jupiter’s orbit is 5.23 AU from our sun, and Saturn’s orbit sits at 9.53 AU. Because of their massive size, most hypergiants only last a few million years before they explode in a supernova. Betelgeuse, the hypergiant in Orion’s constellation, is predicted to go supernova in the next few hundred thousand years. When it does, its brightness will outshine the moon for over a year and even be visible during daylight.
9. Hypervelocity Star
In contrast to the other entries on this list, hypervelocity stars don't have any particularly remarkable features—they are just regular stars, except for the fact that they are racing through space at mind-blowing speeds. With velocities exceeding one or two million miles per hour, hypervelocity stars are the result of stars straying too close to the galactic core, which ejects them at incredibly high speeds. All known hypervelocity stars are traveling at more than twice the escape velocity, meaning they will eventually leave the galaxy and drift alone in the darkness for the rest of eternity.
8. Cepheids
Cepheids, also known as Cepheid Variable Stars, are stars typically 5 to 20 times more massive than our sun. These stars exhibit a regular pulsing pattern, appearing to grow and shrink as they expand and contract due to immense pressure within their dense core. When the pressure builds up, the star enlarges, and as the pressure decreases, it contracts. This cycle of expansion and contraction continues until the star reaches the end of its life.
7. Black Dwarf
If a star lacks the mass to become a neutron star or explode into a supernova, it will eventually turn into a white dwarf. White dwarfs are dense, faint stars that have exhausted their fuel and no longer undergo nuclear fusion. Typically no larger than Earth, white dwarfs cool down over time by emitting electromagnetic radiation. After an incredibly long period, they cool sufficiently to stop emitting light or heat, becoming what is known as a black dwarf—virtually invisible. Black dwarfs represent the final stage in the evolution of many stars, although none currently exist since it takes so long for them to form. Our sun will eventually become one in about 14.5 billion years.
6. Shell Stars
When we imagine stars, we often picture massive, fiery spheres floating through space. However, due to centrifugal force, most stars are slightly oblate, meaning they are somewhat flattened at their poles. While this flattening is minimal in most stars, some stars that spin at incredible speeds become so distorted that they take on a rugby-ball shape. The rapid rotation of these stars also causes them to expel enormous amounts of matter around their equator, forming a 'shell' of gas around them—creating what we call a 'shell star.' In the image above, the faint white ring surrounding the oblate star Alpha Eridan (Achernar) is the 'shell.'
5. Neutron Star
After a star explodes in a supernova, what remains is typically a neutron star. Neutron stars are incredibly compact and dense spheres made entirely of neutrons. These stars are many times denser than the nucleus of an atom, with diameters often less than a dozen kilometers, making them a fascinating product of the laws of physics.
Due to their extraordinary density, any atoms that come into contact with the surface of a neutron star are quickly torn apart. The non-neutron subatomic particles break down into their quarks, which are then reassembled into neutrons. This process releases an enormous amount of energy, enough that a collision between a neutron star and a medium-sized asteroid would generate a gamma-ray burst with more energy than the sun will ever emit in its entire lifespan. This makes neutron stars near our solar system (within a few hundred light years) a serious threat, capable of bombarding Earth with deadly radiation.
4. Dark Energy Star
Given the complexities surrounding our current understanding of black holes, particularly in relation to quantum mechanics, numerous alternative theories have emerged to explain our observations.
One intriguing idea is that of a dark energy star. The hypothesis suggests that when a massive star collapses, it doesn’t form a black hole. Instead, the space-time inside the star transforms into dark energy. Due to quantum mechanics, this star would exhibit an unusual property: outside its event horizon, it would attract matter, but inside, beyond the event horizon, it would repel matter. This is because dark energy possesses 'negative' gravity, similar to how like poles of a magnet push each other away.
Additionally, the theory posits that when an electron crosses the event horizon of a dark energy star, it is converted into a positron, or anti-electron, and ejected. When this antiparticle meets a regular electron, they annihilate each other, releasing a small burst of energy. This process, on a larger scale, could explain the massive radiation emitted from the centers of galaxies, where a supermassive black hole is commonly thought to exist.
In essence, it’s simplest to think of a dark energy star as a black hole that ejects matter and lacks a singularity.
3. Boson Star
In the universe, there are two types of fundamental particles: bosons and fermions. The primary difference between them is that fermions have a half-integer spin, while bosons have an integer spin. All elementary and composite particles, such as electrons, neutrons, and quarks, are classified as fermions, while bosons are the force-carrying particles, like photons and gluons. Unlike fermions, multiple bosons can occupy the same state.
To illustrate this, imagine an analogy: fermions are like buildings, while bosons are like ghosts. You can only have one building at a specific location in space, as it’s impossible to have two buildings coexisting in the same space. However, you can have countless ghosts in the same spot, or even inside the building, as they are immaterial (although bosons do have mass, the analogy holds). There is no limit to how many bosons can occupy the same space.
Currently, all known stars are made up of fermions, but if a stable boson particle with a specific mass were to exist, theoretically, boson stars could also exist. Since gravity is tied to mass, imagine a scenario where an infinite number of particles could coexist at the same point in space. Using the ghost analogy again, picture a billion ghosts, each with a tiny bit of mass, occupying the same spot. This would create a tremendous concentration of mass at one point, generating an immense gravitational pull. Boson stars, therefore, could potentially have infinite mass concentrated in an infinitesimally small space. It’s hypothesized that if boson stars exist, they are likely located at the centers of galaxies.
2. Quasi-Star
“Twinkle, twinkle quasi-star Biggest puzzle from afar How unlike the other ones Brighter than a billion suns Twinkle, twinkle, quasi-star How I wonder what you are.”
– George Gamow, “Quasar” 1964. Hypergiants – the largest of stars – typically collapse into black holes with a mass around ten times that of our sun. This leads to an intriguing question: what could account for the supermassive black holes found at the centers of galaxies, with masses equivalent to a billion suns? No ordinary star is massive enough to create such a behemoth! While one might suggest that these smaller black holes could grow by accreting matter, this is a remarkably slow process. Furthermore, most supermassive black holes are thought to have formed in the universe's early billions of years, meaning conventional black holes would have had too little time to evolve into the giants we observe today. One theory posits that early population III stars, much larger than today's hypergiants and composed solely of helium and hydrogen, rapidly collapsed to form massive black holes, which later merged into the supermassive ones. Another more probable theory suggests that quasi-stars might be the key.
In the early universe, massive clouds of helium and hydrogen roamed the cosmos. If the matter in these clouds collapsed rapidly enough, it could form a massive star with a small black hole at its core – a quasi-star, shining brighter than a billion suns. Ordinarily, this situation would lead to a supernova, with the star’s ‘shell’ and surrounding material being expelled into space. However, if the surrounding matter is dense and large enough, it could resist the blast and begin to fall into the black hole. As the black hole consumes this vast amount of surrounding material, it would grow at an astonishing rate.
To illustrate: imagine having a small bomb encased in cardboard. When the bomb explodes, like a supernova, it blasts the cardboard away, leaving the resulting black hole without matter to consume immediately. But if the cardboard were replaced with thick concrete, the explosion wouldn't scatter the walls – instead, the black hole could immediately begin to consume the material.
1. Iron Star
Stars generate heavier elements through nuclear fusion – a process in which lighter elements combine to form heavier ones, releasing energy in the process. However, the heavier the element, the less energy is released during fusion. Stars typically begin by fusing hydrogen into helium, then helium into carbon, carbon into oxygen, oxygen into neon, neon into silicon, and ultimately, silicon into iron. Fusion of iron consumes more energy than it produces, making it the final stage in any stable nuclear fusion reaction. Most stars die before reaching the point where they start fusing carbon. For those that do, they usually erupt into a supernova shortly after.
An iron star is a star made entirely of iron, yet it paradoxically continues to release energy. How is this possible? Through quantum tunneling. Quantum tunneling is the phenomenon in which a particle passes through a barrier it would otherwise be unable to cross. For instance, if you threw a ball at a wall, it would normally bounce back. However, quantum mechanics allows for a tiny chance that the ball could tunnel through the wall and hit someone on the other side.
This is the essence of quantum tunneling. While the likelihood of such an event occurring is extremely low, on an atomic scale, it happens quite frequently, especially in large objects like stars. Typically, a large amount of energy is required to fuse iron, due to a barrier that resists fusion – meaning more energy is needed than what is released. However, with quantum tunneling, iron can fuse without needing any energy input. To imagine this, think of two golf balls slowly rolling towards each other and merging upon collision. Normally, such fusion would require vast amounts of energy, but quantum tunneling allows it to occur with almost none.
Since iron fusion via quantum tunneling is a rare occurrence, an iron star would need to have an extraordinarily high mass for its fusion reaction to be sustainable. As a result, and due to the relative rarity of iron in the universe, it’s believed that it will take just under 1 Quingentillion years (1 followed by 1503 zeros) before the first iron stars emerge.
