Red Dwarf star and planet. Artists impression (NASA)
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Our Sun is such a familiar sight in the sky that you might think stars like our Sun are common across the Universe. But the most common stars in the Universe are actually much smaller and less massive than the Sun. The Universe is filled with red dwarf stars.
Astronomers categorize a red dwarf as any star less than half the mass of the Sun, down to about 7.5% the mass of the Sun. Red dwarfs can’t get less massive than 0.075 times the mass of the Sun because then they’d be too small to sustain nuclear fusion in their cores.
Red dwarfs do everything at a slower rate. Since they’re a fraction of the mass of the Sun, red dwarfs generate as little as 1/10,000th the energy of the Sun. This means they consume their stores of hydrogen fuel at a fraction of the rate that a star like the Sun goes through. The largest known red dwarf has only 10% the luminosity of the Sun.
And red dwarfs have another advantage. Larger stars, like the Sun, have a core, surrounded by a radiative zone, surrounded by a convective zone. Energy can only pass from the core through the radiative zone by emission and absorption by particles in the zone. A single photon can take more than 100,000 years to make this journey. Outside the radiative zone is a star’s convective zone. In this region, columns of hot plasma carry the heat from the radiative zone up to the surface of the star.
Red dwarfs have no radiative zone, which means that the convective zone comes right down to the star’s core and carries away heat. It also mixes up the hydrogen fuel and carries away the helium by-product. Regular stars die when they use up just the hydrogen in their cores, while red dwarfs keep all their hydrogen mixed up and will only die when they’ve used up every last drop.
With such an efficient use of hydrogen, red dwarf stars with 10% the mass of the Sun are through to live 10 trillion years. Our own Sun will only last about 12 billion or so.
You might be interested to know that the closest star to Earth, Proxima Centauri, is a red dwarf star. Unfortunately, these stars are so small and dim that they can’t be seen without a telescope.
We have written many articles about stars on Universe Today. Here’s an article about how red dwarf stars might have tiny habitable zones. And here’s an article about how they destroy their dust disks.
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A star will live the majority of its live in the main sequence phase. This is where nuclear fusion of hydrogen into helium is happening in its core, and the light pressure of this energy balances out the gravitational collapse of the star. Before a star gets into the main sequence phase, though, it spends some time as a protostar – a baby star.
Stars form when vast clouds of cold molecular hydrogen and helium collapse under mutual gravity. This collapse could have been triggered by a galaxy collision, or the shockwave of a nearby supernova. As the cloud collapses, it breaks into fragments, each of which will eventually become a star of some size.
As the cloud contracts, it begins to increase in temperature. This comes from the conversion of gravitational energy into kinetic energy. The cloud continues to heat up, and the conservation of momentum of all the different particles causes the protostar to spin.
The collapse of the cloud happens fastest at its center, where the material is at the highest density and hottest temperature. Unfortunately these objects are shrouded in dust, and impossible to see with Earth-based observatories. They can be seen in infrared telescopes though, which can pierce through the veil of dust that shrouds them.
As the collapse continues, a disk of gas forms around the protostar, and bi-polar jets blast out from the top and bottom of the star. These produce spectacular shock waves in the clouds.
An object can be considered a protostar as long as material is still falling inward. After about 100,000 years or so, the protostar stops growing and the disk of material surrounding it is destroyed by radiation. It then becomes a T Tauri star, and is visible to Earth-based telescopes.
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Most of the stars in the Universe are in the main sequence stage of their lives, a point in their stellar evolution where they’re converting hydrogen into helium in their cores and releasing a tremendous amount of energy. Let’s example the main sequence phase of a star’s life and see what role it plays in a star’s evolution.
A star first forms out of a cold cloud of molecular hydrogen and helium. Mutual gravity pulls the stellar material together, and this gravitational energy heats it up. The star first goes through a protostar phase for about 100,000 years, and then a T Tauri phase, where it shines only with the energy released from its ongoing gravitational collapse. This second T Tauri phase lasts a further 100 million years or so.
Eventually temperatures and pressures in the core of the star are sufficient that it can ignite nuclear fusion, converting hydrogen atoms into helium. When this process gets going, a star is said to be in the main sequence phase of its life.
In a star like our Sun, the core accounts for about 20% of its radius. It’s inside this region where all the energy of the Sun is released. The energy released in the core must then travel slowly through a radiative zone, where photons of energy are absorbed and then re-emitted. Energy is then carried through a convective zone, where columns of hot plasma carry bubbles of heated gas to the surface of the Sun where it’s released. The material cools down and falls back down inside the Sun where it’s heated up again. This journey can take more than 100,000 years for a single photon to get from the core of a star out to its surface.
Over time, a star slowly uses up the supply of hydrogen in its core, and leftover helium builds up. But the main sequence phase can last a long time. Our Sun has already been in its main sequence for 4.5 billion years, and will probably last another 7.5 billion years before it runs out of fuel.
The smallest red dwarf stars can smolder in the main sequence phase for an estimated 10 trillion years! The largest supergiant stars might only last a few million. It all comes down to mass.
And mass defines how a star comes out of the main sequence phase of its life. For the smallest red dwarf stars, astronomers think they’ll just shut off once they’ve used up all their hydrogen, becoming white dwarfs. More massive stars, with up to 10 solar masses, will go through a red giant phase where they expand many times their original size before collapsing down to the white dwarf. And the most massive stars will just explode as supernovae.
We have written many articles about stars on Universe Today. Here’s an article about the entire life cycle of stars, and different types of stars.
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Look up into the sky and you’ll see the stars twinkling in different colors. Some are dull and red, while others are white and others look bright blue. So how do you get so many different star colors?
The color of a star depends on its surface temperature. Our Sun’s surface temperature is about 6,000 Kelvin. Although it looks yellow from here on Earth, the light of the Sun would actually look very white from space. This white light coming off of the Sun is because its temperature is 6,000 Kelvin. If the Sun were cooler, it would give off light more on the red end of the spectrum, and if the Sun were hotter, it would look more blue.
And that’s just what we see with other stars. The coolest stars in the Universe are the red dwarf stars. These are stars with just a fraction of the mass of our Sun (as low as 7.5% the mass of the Sun). They don’t burn as hot in their cores, and their surface temperature is about 3,500 Kelvin. The light released from their surface looks mostly red to our eyes (although there are different colors mixed up in there too, red is the majority).
This is also the color you see with red giant stars; solar-mass stars that ran out of hydrogen fuel and bloated up many times their original size. The luminosity of the star is spread out over the much larger surface area of the red giant and so they’re cooler,
On the opposite side of the spectrum are blue stars. These are stars with many times the mass of the Sun and so their surface temperatures are much hotter. Blue stars start out above 10,000 Kelvin but they can reach 40,000 Kelvin with the largest hypergiant stars.
We have written many articles about stars on Universe Today. Here’s an article about the biggest stars in the Universe.
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The core of a star is located inside the star in a region where the temperature and pressures are sufficient to ignite nuclear fusion, converting atoms of hydrogen into helium, and releasing a tremendous amount of heat.
The size of the core depends on the mass of the star. For example, our Sun measures 1,391,000 km across and is a fairly normal star. The core of the Sun makes up about 20% of the solar radius; about 278,000 km across. It’s within this region that temperatures reach 15,000,000 Kelvin and nuclear fusion can take place. Fusion doesn’t take place in any other part of the Sun.
As you know, stars can be larger or smaller than the Sun. Larger stars will have larger, hotter cores. The largest stars have cores of 18 million Kelvin, and inside this region hydrogen is fused into helium using a different process called the CNO cycle.
The least massive star capable of sustaining fusion in its core is about 7.5% the mass of the Sun. Below this size, temperatures are too low and you end up with a brown dwarf.
Much like any living being, stars go through a natural cycle. This begins with birth, extends through a lifespan characterized by change and growth, and ends in death. Of course, we’re talking about stars here, and the way they’re born, live and die is completely different from any life form we are familiar with.
For one, the timescales are entirely different, lasting on the order of billions of years. Also, the changes they go through during their lifespan are entirely different too. And when they die, the consequences are, shall we say, much more visible? Let’s take a look at the life cycle of stars.
Molecular Clouds:
Stars start out as vast clouds of cold molecular gas. The gas cloud could be floating in a galaxy for millions of years, but then some event causes it to begin collapsing down under its own gravity. For example when galaxies collide, regions of cold gas are given the kick they need to start collapsing. It can also happen when the shockwave of a nearby supernova passes through a region.
As it collapses, the interstellar cloud breaks up into smaller and smaller pieces, and each one of these collapses inward on itself. Each of these pieces will become a star. As the cloud collapses, the gravitational energy causes it to heat up, and the conservation of momentum from all the individual particles causes it to spin.
Protostar:
As the stellar material pulls tighter and tighter together, it heats up pushing against further gravitational collapse. At this point, the object is known as a protostar. Surrounding the protostar is a circumstellar disk of additional material. Some of this continues to spiral inward, layering additional mass onto the star. The rest will remain in place and eventually form a planetary system.
Depending on the stars mass, the protostar phase of stellar evolution will be short compared to its overall life span. For those that have one Solar Mass (i.e the same mass as our Sun), it lasts about 1000,000 years.
T Tauri Star:
A T Tauri star begins when material stops falling onto the protostar, and it’s releasing a tremendous amount of energy. They are so-named because of the prototype star used to research this phase of solar evolution – T Tauri, a variable star located in the direction of the Hyades cluster, about 600 light years from Earth.
A T Tauri star may be bright, but this all comes its gravitational energy from the collapsing material. The central temperature of a T Tauri star isn’t enough to support fusion at its core. Even so, T Tauri stars can appear as bright as main sequence stars. The T Tauri phase lasts for about 100 million years, after which the star will enter the longest phase of its development – the Main Sequence phase.
Main Sequence:
Eventually, the core temperature of a star will reach the point that fusion its core can begin. This is the process that all stars go through as they convert protons of hydrogen, through several stages, into atoms of helium. This reaction is exothermic; it gives off more heat than it requires, and so the core of a main sequence star releases a tremendous amount of energy.
This energy starts out as gamma rays in the core of the star, but as it takes a long slow journey out of the star, it drops down in wavelength. All of this light pushes outward on the star, and counteracts the gravitational force pulling it inward. A star at this stage of life is held in balance – as long as its supplies of hydrogen fuel lasts.
The life cycle of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years. Credit: ESO/M. Kornmesser
And how long does it last? It depends on the mass of the star. The least massive stars, like red dwarfs with half the mass of the Sun, can sip away at their fuel for hundreds of billions and even trillions of years. Larger stars, like our Sun will typically sit in the main sequence phase for 10-15 billion years. The largest stars have the shortest lives, and can last a few billion, and even just a few million years.
Red Giant:
Over the course of its life, a star is converting hydrogen into helium at its core. This helium builds up and the hydrogen fuel runs out. When a star exhausts its fuel of hydrogen at its core, its internal nuclear reactions stop. Without this light pressure, the star begins to contract inward through gravity.
This process heats up a shell of hydrogen around the core which then ignites in fusion and causes the star to brighten up again, by a factor of 1,000-10,000. This causes the outer layers of the star to expand outward, increasing the size of the star many times. Our own Sun is expected to bloat out to a sphere that reaches all the way out to the orbit of the Earth.
The temperature and pressure at the core of the star will eventually reach the point that helium can be fused into carbon. Once a star reaches this point, it contracts down and is no longer a red giant. Stars much more massive than our Sun can continue on in this process, moving up the table of elements creating heavier and heavier atoms.
White Dwarf:
A star with the mass of our Sun doesn’t have the gravitational pressure to fuse carbon, so once it runs out of helium at its core, it’s effectively dead. The star will eject its outer layers into space, and then contract down, eventually becoming a white dwarf. This stellar remnant might start out hot, but it has no fusion reactions taking place inside it any more. It will cool down over hundreds of billions of years, eventually becoming the background temperature of the Universe.
Eta Carinae Credit: Gemini Observatory artwork by Lynette Cook
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Stars can range in temperature, from the relatively cool red dwarfs to superhot blue stars. So what is the hottest star in the Universe?
First, let’s talk a bit about temperature. The color of a star is a function of its temperature. If a star looks red, that means its surface temperature is approximately 2,500 Kelvin. Just for comparison, our Sun, which actually looks white from space, measures about 6,000 Kelvin. The hotter the star, the further up the spectrum you go. The hottest stars are the blue stars. A star appears blue once its surface temperature gets above 10,000 Kelvin, or so, a star will appear blue to our eyes.
So the hottest stars in the Universe are going to be a blue star, and we know they’re going to be massive. So the question is, how massive can stars get? One example is the star Rigel, in the constellation Orion. Rigel is thought to have 17 times the mass of the Sun, and puts out 40,000 times the luminosity of the Sun. It’s surface temperature is a mere 11,000 Kelvin. Another star in Orion, Bellatrix, has a temperature of 21,500 Kelvin. That’s even hotter.
But the hottest known stars in the Universe are the blue hypergiant stars. These are stars with more than 100 times the mass of the Sun. One of the best known examples is Eta Carinae, located about 7,500 light-years from the Sun. Eta Carinae could be as large as 180 times the radius of the Sun, and its surface temperature is 36,000-40,000 Kelvin.
Just for comparison, 40,000 Kelvin is about 72,000 degrees F.
So it’s the blue hypergiants, like Eta Carinae, which are probably the hottest stars in the Universe.
We have written many articles about stars on Universe Today. Here’s an article about how Eta Carinae is almost ready to explode as a supernova. And here’s a link to a nice photo of the nebula around Eta Carinae.
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The color of a star is defined by its temperature. The coolest stars are red, while the hottest stars are blue. And the temperature of a star is defined by its mass. The most massive stars in the Universe are the blue supergiant stars; then can have more than 20 times the mass of the Sun. Blue giant stars are very hot, with surface temperatures of 20,000-50,000 Kelvin. Just for comparison, our own Sun is only 6,000 Kelvin.
Blue supergiant star have extremely high masses, sometimes with dozens of times the mass of the Sun. They form in the largest, most active star forming regions where large amounts of mass can come together to form the biggest stars: star clusters, the arms of spiral galaxies and in irregular galaxies.
Perhaps the best known example of a blue supergiant star is Rigel, located in the constellation Orion. It has about 20 times the mass of the Sun, and puts out 60,000 times as much energy.
Blue supergiants can turn into red supergiants and vice versa. When the star is smaller and more compact, its luminosity is contained over a smaller surface area and so its temperature is much hotter; this is the blue supergiant phase. These stars can then puff up expanding to a much larger size, spreading their luminosity over a much larger area. Then they become red supergiant stars, and appear the cooler red color. Astronomers think supergiants can fluctuate back and forth between red and blue supergiant, puffing off an outer layer of material with each contraction.
Eventually a supergiant runs out of material to continue supporting fusion in its core, and will detonate as a supernova – one of the brightest explosions in the Universe.
We have written many articles about stars on Universe Today. Here’s an article that talks about the constellation Orion, including the star Rigel, and here’s a nice picture of Rigel passing behind Saturn.
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Stars come in many shapes and sizes and they come in many colors. Some of the hottest stars in the Universe are blue giant stars. You see, the color of a star is defined by its temperature; the coolest stars are red, while the hottest ones appear blue. And the temperature of a star comes from its mass. The more massive a star, the hotter it’s going to be. Stars don’t get more more massive or hot than blue giant stars.
Blue giants blaze with a surface temperature of 20,000 Kelvin or more, and are extremely luminous. Just for comparison, a star like our Sun only has a surface temperature of about 6,000 Kelvin. A blue giant star can put out 10,000 times as much energy as the Sun. Astronomers categorize blue giants as type O or B stars, belonging to the luminosity class III. The can reach an absolute magnitude of -5 or -6.
The true monsters of the Universe are blue supergiant stars, like Rigel. These can be a blue star with surface temperatures of 20,000 – 50,000 Kelvin and can be 25 times larger than the Sun. Because they’re so large, and burn so hot, they use up their fuel very quickly. A middle-sized star like our Sun might last for 12 billion years, while a blue supergiant will detonate with a few hundred million years. The smaller stars will leave neutron stars or black holes behind, while the largest will just vaporize themselves completely.
We have written many articles about stars on Universe Today. Here’s an article that looks for the biggest star in the Universe.
The constellation of Vulpecula is unusual, because it did not belong originally to those created by Ptolemy – but to the works of Johannes Hevelius. Vulpecula was included in Firmamentum Sobiescianum, a 56 page atlas created by Hevelius, which outlined seven new constellations which survived time – and many which did not. Positioned north of the ecliptic plane, it spans 268 square degrees of sky, ranking 55th in constellation size. It has 5 main stars in its asterism and 33 Bayer Flamsteed designated stars within its confines. Vulpecula is bordered by the constellations of Cygnus, Lyra, Hercules, Sagitta, Delphinus and Pegasus. It is best seen at culmination during the month of September.
Since Vulpecula is considered a “modern” constellation, there is no mythology associated with it – although the stellar pattern was very visible to the ancient Greeks and Romans. Late in 17th century, astronomer Johannes Hevelius created the constellation of Vulpecula when he was preparing his own set of star charts known as Firmamentum Sobiescianum At the time, he named it “Vulpecula Cum Ansere” which literally translated to the little fox with the goose – and he illustrated it as a fox with a goose caught in its jaws. At the time, Hevelius did not consider it to be two separate constellations – yet it was later divided into two halves – Vulpecula and Anser “The Goose”. When star charts were once again consolidated, the constellations merged again to be known under to modern named assigned to it by the International Astronomical Union as Vulpecula, yet the primary star retains a reminder be being properly named Anser.
Let’s begin our binocular tour of Vulpecula with a look at the Alpha (“a”) star – Anser. Its name literally translates to “goose”, but this class M giant star is anything but flighty. Residing 297 light years from Earth, Anser puts out 390 times more light energy than our Sun from a size about 45 times larger. It may have a dead helium core about to begin hydrogen fusion – and it may have a dead carbon-oxygen core awaiting a second brightening before turning K class. If you’ve notice another nearby star – good for you! Although it’s only a line of sight companion, 8 Vulpeculae makes checking out Anser a real treat!
Now head on to Collinder 399. This wonderful asterism is often called “Brocchi’s Cluster” or the “Coathanger” and it’s a splendid object in binoculars or a rich field telescope. This unique collection of stars was known as far back as 964 AD when astronomer Al Sufi recorded it, and it was independently rediscovered by Giovanni Hodierna in the seventeenth century. In the 1920s, D. F. Brocchi, an amateur astronomer and chart maker for the American Association of Variable Star Observers, created a map of this object for use in calibrating photometers. Thanks to its expansive size of more than 60 arc minutes, it escaped the catalogues of both Messier and Herschel. Only around a half dozen stars share the same proper motion, which may make it a cluster much like the Pleiades, but studies suggest it is merely an asterism…but one with two binary stars at its heart.
Our next target is the magnificent Messier 27 (RA 19 : 59.6 Dec +22 : 43). This incredible planetary nebula appears like a pale green apple core and is unquestionably the brightest study of its kind. Easily located around a finger-width north of Gamma Sagittae, it’s not the largest of all planetaries but is the largest of its kind on the Messier list. M27’s expanse and luminosity suggest that it is quite close to our own system. Some think it difficult to find, but there is a very simple trick. Look for the primary stars of Sagitta just to the west of bright Albireo. Make note of the distance between the two brightest and look exactly that distance north of the “tip of the arrow” and you’ll find M27.
Discovered in 1764 by Messier in a 3.5 foot focal length telescope, I discovered this 48,000 year old planetary nebula for the first time in a 4″ telescope. I was hooked immediately. Here before my eager eyes was a glowing green “apple core” which had a quality about it that I did not understand. It somehow moved… It pulsated. It appeared “living.” For many years I quested to understand the 850 light-year distant M27, but no one could answer my questions. I researched and learned it was made up of doubly ionized oxygen. I had hoped that perhaps there was a spectral reason to what I viewed year after year – but still no answer. Like all amateurs, I became the victim of “aperture fever” and I continued to study M27 with a 12″ telescope, never realizing the answer was right there – I just hadn’t powered up enough.
Several years later while studying at the Observatory, I was viewing through a friend’s identical 12″ telescope and, as chance would have it, he was using about twice the magnification that I normally used on the “Dumbbell.” Imagine my total astonishment as I realized for the very first time that the faint central star had an even fainter companion that made it seem to wink! At smaller apertures or low power, this was not revealed. Still, the eye could “see” a movement within the nebula – the central, radiating star and its companion. Do not sell the Dumbbell short. It can be seen as a small, unresolved area in common binoculars, easily picked out with larger binoculars as an irregular planetary nebula, and turns astounding with even the smallest of telescopes. In the words of Burnham, “The observer who spends a few moments in quiet contemplation of this nebula will be made aware of direct contact with cosmic things; even the radiation reaching us from the celestial depths is of a type unknown on Earth…”
Ready for a galactic star cluster for both binoculars and a small telescope? The return to Alpha and begin about two fingerwidths southeast and right on the galactic equator you’ll find NGC 6823 (RA 19 : 43.1 Dec +23 : 18). The first thing you will note is a fairly large, somewhat concentrated magnitude 7 open cluster. Resolved in larger telescopes, the viewer may note these stars are the hot, blue/white variety. For good reason. NGC 6823 only formed about 2 billion years ago. Although it is some 6000 light-years away and occupies around 50 light-years of space, it’s sharing the field with something more – a very large emission/reflection nebula, NGC 6820. In the outer reaches of the star cluster, new stars are being formed in masses of gas and dust as hot radiation is shed from the brightest of the stellar members of this pair. Fueled by emission, NGC 6820 isn’t always an easy visual object – it is faint and covers almost four times as much area as the cluster. But trace the edges very carefully, since the borders are much more illuminated than the region of the central cluster. Take the time to really observe this one! Its processes are very much like those of the “Trapezium” area in the Orion nebula. Be sure to mark your observing notes. NGC 6823 is Herschel VII.18 and NGC 6820 is also known as Marth 401!
If you’d like to try something new, return to M27 and head 2 degrees west-northwest to find NGC 6830 (RA 19 : 51.0 Dec +23 : 04). This rich 7.9 magnitude, cross-shaped open cluster is a real treat. Continue another 2 degrees in the same direction to pick up 7.1 magnitude cluster NGC 6823. Those with large telescopes should look for a faint sheen of nebulosity associated with this youthful open cluster!
Now let’s work on a pair of open star clusters for both binoculars and small telescopes, starting with NGC 6885 (RA 20 : 12.0 Dec +26 : 29). This little 6th magnitude sparkle of stars includes that bright O class star you can see visually and is also known as Caldwell 37. In binoculars you’ll see another compression nearby listed as NGC 6882 (RA 20 : 11.7 Dec +26 : 33). While it doesn’t contain a bright and splashy star like its neighbor, NGC 6882 is a nice ring shaped collection!
Our last official target in Vulpecula is superb galactic star cluster NGC 6940 (RA 20 : 34.6 Dec +28 : 18). This 6th magnitude, 31 arc minute cloud of stars is sure to please anyone with any size binoculars or telescope. The more aperture you have – the more stars you resolve! Discovered by Sir William Herschel in 1784 and logged as H VIII.23, this intermediate-aged galactic cluster will blow your mind in large aperture. Although visible in binoculars, as aperture increases the field explodes into about 100 stars in a highly compressed, rich cloud. Although not visited often, NGC 6940 is on many observing challenge lists. Use low power to get the full effect of this stunning starfield!
While NGC 6834 (RA 19 : 52.2 Dec +29 : 25) is officially listed as Cygnus, why not visit anyway? You’re in the neighborhood! It’s a very rich and compact small star cluster that’s a worthy challenge to pick out of the Milky Way star field in a telescope!
