Artist's conception of the Milky Way galaxy. Credit: Nick Risinger
When you look up at the night sky, assuming conditions are just right, you might just catch a glimpse of a faint, white band reaching across the heavens. This band, upon closer observation, looks speckled and dusty, filled with a million tiny points of light and halos of glowing matter. What you are seeing is the Milky Way, something that astronomers and stargazers alike have been staring up at since the beginning of time.
But just what is the Milky Way? Well, simply put, it is the name of the barred spiral galaxy in which our solar system is located. The Earth orbits the Sun in the Solar System, and the Solar System is embedded within this vast galaxy of stars. It is just one of hundreds of billions of galaxies in the Universe, and ours is called the Milky Way because the disk of the galaxy appears to be spanning the night sky like a hazy band of glowing white light. Continue reading “What is the Milky Way?”
June 21st, 2011 is Summer Solstice – the longest day of the year.
This is the time when the Sun is at its highest or most northerly point in the sky in the Northern Hemisphere and when we receive the most hours of daylight. If you live in the Southern Hemisphere it is the reverse, so you will be having “Winter Solstice.”
Also known as “Midsummer” the Summer Solstice gets its name from the Latin for sol (sun) and sistere (to stand still). The Sun reaches its most Northerly point and momentarily stands still before starting its journey South in the sky again until it reaches its most Southerly point “Winter Solstice”, before repeating the cycle. This is basically how we get our seasons.
It’s not actually the Sun that moves North or South over the seasons although it may appear so. It’s the Earths axial tilt that causes the Sun to change position in the sky as the Earth orbits the Sun throughout the year.
The angle of the Sun and the Earth's seasons. Image credit: NASA
Summer Solstice/ Midsummer is steeped in ancient folklore especially in Northern Europe with the most famous place directly related to it being Stonehenge, where the sun has been worshiped for thousands of years.
Stonehenge Credit: bistrochic.net
The Sun reaches its most Northerly point in the sky at 17:16 UTC momentarily and from that point forward starts to make its way South. This means the days will get shorter and shorter until Winter Solstice in December.
Today, we take it for granted that the Sun produces energy via nuclear fusion. However, this realization only came about in the early 1900’s and wasn’t confirmed until several decades later (see the Solar Neutrino Problem). Prior to that, several other methods of energy production had been proposed. These ranged from burning coal to a constant bombardment of comets and meteors to slow contraction. Each of these methods seemed initially plausible, but when astronomers of the time worked out how long each one could sustain such a brightness, they came up against an unlikely opponent: Charles Darwin.
In a “Catholic Magazine and Review” from 1889, known as The Month, there is a good record of the development of the problem faced in an article titled “The Age of the Sun and Darwinism”. It begins with a review of the recently discovered Law of Conservation of Energy in which they establish that a method of generation must be established and that this question is necessarily entangled with the age of the Sun and also, life on Earth. Without a constant generation of energy, the Sun would quickly cool and this was known to be unlikely due to archaeological evidences which hinted that the Sun’s output had been constant for at least 4,000 years.
While burning coal seemed a good candidate since coal power was just coming into fashion at the time, scientists had calculated that even burning in pure oxygen, the Sun could only last ~6,000 years. The article feared that this may signal “the end of supplies of heat and light to our globe would be very near indeed” since religious scholars held the age of the Earth to be some “4000 years of chronological time before the Christian era, and 1800 since”.
The bombardment hypothesis was also examined explaining that the transference of kinetic energy can increase temperatures citing examples of bullets striking metal surfaces or hammers heating anvils. But again, calculations hinted that this too was wrong. The rate with which the Sun would have to accumulate mass was extremely high. So much so that it would lead to the “derangement of the whole mechanism of the heavens.” The result would be that the period of the year over the past ~6,000 years would have shortened by six weeks and that the Earth too would be constantly bombarded by meteors (although some especially strong meteor showers at that time lent some credence to this).
The only strong candidate left was that of gravitational contraction proposed by Sir William Thomson (later Lord Kelvin) and Hermann von Helmholtz in a series of papers they began publishing in 1854. But in 1859, Darwin published the Origin of Species in which he required an age of at least two billion years. Thomson’s and Helmholtz’s hypothesis could only support an age of some tens of millions of years. Thus astronomy and biology were brought head to head. Darwin was fully aware of this problem. In a letter to a friend, he wrote that, “Thomson’s views of the recent age of the world have been for some time one of my sorest troubles”.
To back the astronomers was the developing field of spectroscopy in which they determined that the sun and other stars bared a strong similarity to that of nebulae. These nebulae could contract under their own gravity and as such, provided a natural establishment for the formation of stars, leading gracefully into the contraction hypothesis. Although not mentioned in the article, Darwin did have some support from geologists like Charles Lyell who studied the formation of mountain ranges and also posited an older Earth.
Some astronomers attempted to add other methods in addition to gravitational contraction (such as tidal friction) to extend the age of the solar system, but none could reach the age required by Darwin. Similarly, some biologists worked to speed up evolutionary processes by positing separate events of abiogenesis to shave off some of the required time for diversification of various kingdoms. But these too could not rectify the problem.
Ultimately, the article throws its weight in the camp of the doomed astronomers. Interestingly, much of the same rhetoric in use by anti-evolutionists today can be found in the article. They state, “it is not surprising to find men of science, who not only have not the slightest doubt about the truth of their own pet theories, but are ready to lay down the law in the realms of philosophy and theology, in science which with, to judge from their immoderate assertions, their acquaintance is of the most remote? Such language is to be expected from the camp-followers in the army of science, who assurance is generally inversely proportional to their knowledge, for many of those in a word who affect to popularize the doctrine of Natural Selection.”
In time, Darwin would win the battle as astronomers would realize that gravitational contraction was just the match that lit the fuse of fusion. However, we must ask whether scientists would have been as quickly able to accept the proposition of stellar fusion had Darwin not pointed out the fundamental contradiction in ages?
The conveyor belt of the Sun - a large flow of plasma that circulates under the surface - may be responsible for the duration of solar cycles. Image Credit: Science@NASA
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The Sun seems to finally be waking up in earnest from the long slumber of the past cycle. Solar cycles tend to last on average about 11 years, but the last cycle – solar cycle 23 – was 12.5 years long. The cause of the most recent lull in the Sun’s activity is somewhat enigmatic, but it may be explained by the “conveyor belt” of plasma that circulates in the Sun’s chromosphere and photosophere. Just how far this conveyor belt of plasma extends underneath the Sun may heavily influence the duration of solar cycles.
In a recent paper published in Geophysical Research Letters, Dr. Mausumi Dikpati of the High Altitude Observatory National Center for Atmospheric Research in Boulder, Colorado and her team modeled data from the Mount Wilson Observatory for the duration of the last solar cycle. When they analyzed and modeled surface Doppler measurements of the flow of plasma currents that course underneath the surface of the Sun, they discovered that the flow extended all the way to the poles.
This is in contrast to data from previous, average-length solar cycles, in which the meridional plasma flow – or the Sun’s conveyor belt – flowed only to about 60 degrees latitude. This flow is not unlike thermohaline circulation here on Earth, in which the ocean transports heat around the globe.
Dr. Dikpati said in an email interview, “This is the first time that the Sun’s conveyor-belt has been measured accurately enough for two consecutive cycles (cycles 22 spanning approximately 1986-1996.5 and cycle 23 spanning 1996.5-2009). From these data we now know that cycle 22 had a shorter conveyor-belt reaching only to 60-degree latitude, while cycle 23 had a long conveyor-belt extending all the way to the pole.”
The cycles of the Sun are intricately linked to the magnetic field permeating our nearest star. Gigantic loops of the magnetic field of the Sun are what cause sunspots, and as the contours of the magnetic field change over the cycle of the Sun, more or fewer sunspots are seen, as well as solar flares and other activity. There is always a lack of sunspots between the cycles, but the minimum at the end of cycle 23 was unusually long.
The conveyor belt of plasma flowing in the chromosphere and photosphere essentially drags along with it the magnetic flux of the Sun. Because the extent of the conveyor belt reached a higher latitude, it took the magnetic flux longer to return to the equator, resulting in the delay of sunspots marking the onset of cycle 24.
Dr. Dikpati and her team determined that it wasn’t the speed of the flow of plasma conveyor belt that lengthened the solar cycle, but the extent into higher latitudes, and slower return to the equator. Though the speed of the conveyor belt was a bit higher than usual over the past five years, it also stretched much further than during a normal cycle.
Dr. Dikpati said of using data from previous solar cycles to better refine their model of the conveyor belt:
From the same data source (Mount Wilson data from Roger Ulrich) there is evidence of a short conveyor-belt in cycles 19, 20 and 21 also. All these cycles had periods (10.5 years) like cycle 22. Back beyond that we are hoping that others in the community will search for evidence of the latitudinal extent of the conveyor-belt in even earlier cycles. In fact, theory of the conveyor-belt in high-latitudes indicates that a shorter conveyor belt should be more common in the Sun, rather this long conveyor belt in cycle 23 may be the exception. There is already evidence from Mount Wilson data that, at the start of cycle 24, the conveyor-belt is shortening again, suggesting that cycle 24 is going to be more like cycles 19 – 22 in length.
By getting a better model of the interplay between the plasma flow and the Sun’s magnetic field, solar scientists may be able to better predict and explain the length of future and past solar cycles.
Dr. Dikpati said, “The conveyor belt also governs the memory of the Sun about its past magnetic features. This is an important ingredient for building prediction models for solar cycles.”
A sun dog is an atmospheric phenomenon where you can see additional bright patches in the sky on either side of the Sun. Sometimes you just see bright spots, and sometimes you can actually see an arc or even a halo around the Sun. These are all related to sun dogs, and have to do with very specific atmospheric conditions. If you’ve ever seen a sun dog, you were very lucky, and they only occur rarely.
Sun dogs occur because of sunlight refracting through ice crystals in the atmosphere. The crystals cause the sunlight to bend at a minimum angle of 22°. All of the crystals are refracting the Sun’s rays, but we only see the ones which are bent towards our eyes. Because this is the minimum, the light looks more concentrated starting at 22° away from the Sun; about 40 times the size of the Sun in the sky. At this 22° point you can get arcs, a halo, or just bright spots in the sky.
They can occur at any time of the year and from any place on Earth; although, they’re easiest to see when the Sun is lower on the horizon. As the Sun rises, the sun dog can actually drift away from the 22° point. Eventually the Sun gets so high that the sun dog disappears entirely.
There are no set colors with sun dogs. The light from the Sun is being refracted equally by the ice crystals and so we don’t see the colors broken up as we do with a rainbow.
We’ve written several articles about the Sun for Universe Today. Here’s an article about a ring around the Sun, and here’s an article about rings around the Moon.
Green Flash in Santa Cruz, California. Image credit: Mila Zinkova
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Have you ever heard of a green flash sunset? You might think it’s a myth, but this is a real phenomenon that you can see if the conditions are just right. If you’re watching the Sun dip down on the horizon you might see a green dot appear just above the Sun for just a second. That’s a green flash sunset, and if you saw one, you’re a very lucky person.
Green flashes can occur at sunrise or sunset, and to see one, you need to have an unobstructed view to the horizon. They occur because the light from the Sun is refracted – or bent – as it passes through the Earth’s atmosphere, following the curvature of the Earth. Higher frequency light (bluer light) is bent more than lower frequency light. This is happening all the time, but we’re seeing all the colors of the light spectrum at the same time. But when the Sun is right at the horizon, the redder hues of the color spectrum are blocked by the horizon of the Earth, while the higher frequency wavelengths are still following the curve of the Earth. While the redder light is blocked, the green and blue light is still visible, so we see the green flash.
There are actually a few different kinds of green flashes that can occur. The most common example is an inferior-mirage flash, where a dot of green light appears on top of the Sun just as it’s gone below the horizon. But you can also get a situation where a portion of the Sun’s upper edge turns slightly green, or even a green beam of light appears above the Sun.
We’ve written a few articles about sunsets for Universe Today. Here’s an article about green flashes, and here are some cool pictures of sunsets seen from other worlds.
Plasma on the surface of the Sun. Image credit: Hinode
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The Sun may look like just a mass of incandescent gas (plasma, really), but it’s actually broken up into layers. The chromosphere is relatively thin region of the Sun that’s just above the photosphere.
The photosphere is the region of the Sun that we see. It measures an average temperature of almost 5,800 kelvin and produces the visible radiation. This is the point where photons generated inside the Sun can finally leap out into space. The chromosphere measures just 2,000 km, and it’s just outside the photosphere.
Even though it’s very thin, the chromosphere changes dramatically in density, from the top down to the photosphere, the density of the chromosphere increases by a factor of 5 million. The upper boundary of the chromosphere is the called the solar transition region, above which is known as the corona.
One surprising mystery is that the chromosphere is actually hotter than the photosphere. While the photosphere hovers around 5,800 kelvin, the temperature of the chromosphere varies between 4,500 K and 20,000 K. Even though it’s more distant from the center of the Sun, the chromosphere is hotter than the photosphere. Astronomers think turbulence in the Sun’s atmosphere might somehow cause this extra heating.
The chromosphere is difficult to see without special equipment because the light from the much brighter photosphere washes it out. It has a reddish color, but you can only really see it during a total solar eclipse.
One of the recognizable features of the chromosphere are spicules. These are fingers of gas that kind of look like grass growing on the surface of the Sun. These can rise up in the chromosphere and then disappear again within 10 minutes.
Plasma on the surface of the Sun. Image credit: Hinode
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The Sun is the hottest place in the Solar System. The surface of the Sun is a mere 5,800 Kelvin, but down at the core of the Sun, the temperatures reach 15 million Kelvin. What’s going on, why is the Sun hot?
The Sun is just a big plasma ball of hydrogen, held together by the mutual gravity of all its mass. This enormous mass pulls inward, trying to compress the Sun down. It’s the same reason why the Earth and the rest of the planets are spheres. As the pull of gravity compresses the gas inside the Sun together, it increases the temperature and pressure in the core.
If you could travel down into the Sun, you’d reach a point where the pressure and temperature are enough that nuclear fusion is able to take place. This is the process where protons are merged together into atoms of helium. It can only happen in hot temperatures, and under incredible pressures. But the process of fusion gives off more energy than it uses. So once it gets going, each fusion reaction gives off gamma radiation. It’s the radiation pressure of this light created in the core of the Sun that actually stops it from compressing any more.
The Sun is actually in perfect balance. Gravity is trying to squeeze it together into a little ball, but this creates the right conditions for fusion. The fusion releases radiation, and it’s this radiation that pushes back against the gravity, keeping the Sun as a sphere.
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When was the Sun discovered? Obviously the Sun is such an important feature in our lives, and the absolute necessity to all life on Earth. It’s kind of impossible to say when the Sun was discovered, since the first life forms on Earth probably relied on its energy. Humans have been well aware of the Sun for tens of thousands of years, and before modern astronomy had no idea what it was.
So perhaps a better question might be, when did we realize that the Sun is a star?
The Sun is incredibly important to our lives. When the Sun is in the sky, we have day. And when the Sun is below the horizon, we have night. Our biological clocks are programmed on it, and we life our lives by this routine. Ancient peoples thought the Sun was some kind of deity, and many civilizations – like the Inca in South America – worshipped it.
The Greek philosopher Anaxagoras first proposed that the Sun was a burning ball of fire, larger than a Greek Island, and not the chariot of a god. And other astronomers were able to calculate the distance to the Sun with surprising accuracy. In the modern scientific era Lord Kelvin proposed that the Sun was ball of hot liquid that was slowly cooling. But it wasn’t until the early 20th century that scientists were finally able to figure out what the source of the Sun’s energy is.
Ernest Rutherford proposed that the Sun’s heat came from radioactive decay, and it was Albert Einstein who used his famous mass-energy equation (E=mc2) to suggest that the Sun was converting mass into energy. And finally, the theoretical concept of fusion was created in the 30s by Subrahmanyan Chandrasekhar and Hans Bethe. They were able to calculate the actual fusion reactions in the Sun that convert hydrogen into helium.
I would say then, that the Sun was really discovered in the 1930s, when astrophysicists finally understood the mechanisms working inside the Sun that gave off so much energy.
We have written many articles about the Sun for Universe Today. Here’s an article about how big the Sun is, and here’s an article about the Sun’s future.
If you’d like more information about the Sun, check out NASA’s website for the SOHO spacecraft mission.
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.
