Category: Guide to Space

The Earth isn’t flat, that’s for sure. And if you look at a photograph, the Earth really looks round. But how round is it?

The actual shape of the Earth is actually an oblate spheroid – a sphere with a bulge around the equator. The Earth is bulged at its equator because it’s rapidly rotating on its axis. The centripetal force of the rotation causes the regions at the equator to bulge outward. And it actually makes a pretty big difference. The diameter of the Earth, measured across the equator is 43 km more than when you measure the diameter of the Earth from pole to pole.

This bulge has some interesting implications. For example, it means that the point on Earth furthest from the center isn’t actually Mount Everest, but Mount Chimborazo in Ecuador. Only because Chimborazo is closer to the Earth’s equator.

So how smooth is the Earth. When billiard balls are manufactured, they aim for a tolerance of 0.22%. The Earth has a tolerance of 0.17%, so it’s actually smoother than a billiard ball. If you could hold the Earth in your hands, it would feel smoother than a billiard ball.

But the Earth definitely isn’t flat.

We have written many articles about the Earth for Universe Today. Here’s a cool article about looking at the Earth as if it’s an extrasolar planet.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

[/caption]

If you’ve seen an image of the Milky Way from above or below, you will certainly notice that it has a spiral structure. Not all galaxies are created equal, though, as there are many, known as elliptical galaxies, that are blob-like, while others have irregular shapes. Ours is of a class of galaxies called barred spirals, because it has a rectangular bar in the middle of the galactic disk.

The Milky Way has four main spiral arms: the Norma and Cygnus arm, Sagittarius, Scutum-Crux, and Perseus. The Sun is located in a minor arm, or spur, named the Orion Spur. The galactic disk itself is about 100,000 light years across, and the bar at the center is estimated to be about 27,000 light years long.

Why is the Milky Way a spiral? This is due to its rotation, or rather, the rotation of matter inside the galactic disk around the center. It’s not as if the stars themselves stay in the spiral arms, and rotate around the center of the galaxy, though: if they did this, the arms would wind in tighter and tighter over time (2 billion years or so), since the stars in the center revolve faster than those further out.

The spirals are actually what is called a density wave or standing wave. The best way to describe this is the analogy of a traffic jam: cars travel on a busy road in a city, bunching up in jams over the course of a day at certain sections. But the cars move through the jam eventually, and other cars pile up behind them in the jam. The wave is at a certain location, with bunches of matter piling up there for a while, then moving on to be replaced by other matter. As dust and gas is compressed in the spirals, it is heated up and results in the formation of new stars. This star formation makes the trailing edge of the spiral brighter, and places the density wave “ahead”, where dimmer, redder stars are starting to be compressed.

When you see an image of the Milky Way like the one above, it’s not actually a photo of our galaxy. Since we inhabit the disk and have no way (currently) of going above or below, images of the Milky Way are generated by computers or artists. Astronomers have determined that the Milky Way is a spiral galaxy by mapping the movements of stars and hydrogen clouds in the disk.

The Milky Way is far from being the only spiral galaxy in the Universe. To view images of other spiral galaxies, go to the aptly-named Spiral Galaxies website, or NASA’s Astronomy Picture of the Day Spiral Galaxy Index.

To learn more about the Milky Way, check out Episode 99 of Astronomy Cast, or visit the rest of the Milky Way section in the Guide to Space.

Source: University of Wisconsin-Madison News

Frequently, readers send us questions here on Universe Today. One very good question is ”why does Jupiter have the Great Red Spot?” The short answer is that the Great Red Spot is a storm that has been raging since the 1600s, but a short answer does not tell the whole story. Read on for a much more detailed accounting.

The Great Red Spot (GRS) is an anti-cyclonic(rotates counter-clockwise) storm that is located 22° south of Jupiter’s equator. The storm has lasted an estimated 346 years, but many scientists believe that it is much older. The storm is known to have been larger than 40,000 km in diameter at one time and can be easily seen with large backyard telescopes. Currently it measures approximately 24–40,000 km east–to–west and 12–14,000 km north–to–south. The GRS is large enough to envelope two to three Earths. Despite the GRS’s enormous size, it is shrinking. In 2004, it had about half the longitudinal size that it had a century ago. Some scientist estimate that if it continues to shrink at its current rate, it could become circular by 2040. A study by scientists at the University of California, Berkeley showed that the GRS lost 15 percent of its diameter along its major axis between 1996 and 2006. Xylar Asay-Davis, a team member on the study, noted that the spot is not disappearing because ”[v]elocity is a more robust measurement because the clouds associated with the Red Spot are also strongly influenced by numerous other phenomena in the surrounding atmosphere.”

Infrared data indicates that the GRS is colder and located at a higher altitude than most of Jupiter’s other clouds. The cloudtops of the GRS are about 8 km above the surrounding clouds. The storm is held in place by an eastward jetstream to its south and a very strong westward jetstream to its north. Winds around the edge of the GRS peak at 432 km/h, but winds within the storm seem to nearly none existent, with little inflow or outflow. In 2010, astronomers imaged the GRS in the far infrared and found that its central(reddest) region is warmer than its surroundings by about 4 K. The warm airmass is located in the upper troposphere. This warm central spot slowly rotates in the opposite direction of the remainder of the storm and could be a remnant of air flow in the center.

Alright, so why is the storm red? The exact cause of the coloring has not been proven, but…lab experiments support the theory that the color is caused by complex organic molecules, red phosphorus, or another sulfur compound that are pulled from deeper within Jupiter. The color of the GRS varies. At times it is brick-red, fading to a pale salmon, and even white. The spot occasionally disappears from the visible spectrum and can only be seen as the Red Spot Hollow; its niche in the South Equatorial Belt(SEB). The visibility of GRS is apparently coupled to the appearance of the SEB. If the SEB is bright white, the spot tends to be dark. When it is dark, the GRS is usually light. The periods that the color changes last and occur on an unpredictable schedule.

As you can see, the answer to ”why does Jupiter have the Great Red Spot?” has been well researched by NASA and other space agencies. While the answer is not crystal clear at this time, future missions to the planet are designed to better study the atmosphere; hopefully, rendering the answers scientists seek.

We have written many articles about Jupiter for Universe Today. Here are some interesting facts about Jupiter, and here’s an article about the color of Jupiter.

If you’d like more information on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded an episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Sources:
http://www.nasa.gov/multimedia/imagegallery/image_feature_413.html
http://www.nasa.gov/centers/goddard/news/topstory/2006/little_red_spot.html

The first rocket ever sent to space probably carried bacteria or some other accidental passenger. But the first animals ever intentionally sent into space were fruit flies launched aboard a V2 rocket in 1947. US scientists were studying the effects of radiation at high altitude.

A rhesus monkey called Albert 1 became the first monkey launched into space on June 11, 1948; also on board a US-launched V2 rocket.

These were just suborbital flights, though. The first animal to actually go into orbit was the dog Laika, launched on board the Soviet Sputnik 2 spacecraft on November 3, 1957. Unfortunately, Laika died during the flight.

At least 10 more dogs were launched into space and on sub-orbital flights by the Soviets until April 12, 1961, when Yuri Gagarin became the first human in space.

Since those first historic launches, many monkeys, chimpanzees, rats, mice, frogs, spiders, cats and even a tortoise were launched into space.

Read more about Laika’s mission in this article.

Want more resources on space? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Space Place.

We have also recorded many episode of Astronomy Cast about space. Episode 100 is all about rockets, and Episode 84 is about getting around the Solar System.

Our Earth keeps us very safe from a dangerous Universe that’s always trying to kill us in new and interesting ways.

Risk: Cosmic rays are high energy particles fired at nearly the speed of light by the Sun, supermassive black holes and supernovae. They have the ability to blast right through your body, damaging DNA as they go. Long term exposure to cosmic rays increases your chances of getting cancer. Fortunately, we have our atmosphere to protect us. As cosmic rays crash into the atmosphere, they collide with the oxygen and nitrogen molecules in the air.

Risk: Gamma rays and X-rays. As you know, radiation can damage the body. Just a single high-energy photon of gamma rays can cause significant damage to a living cell. Once again, though, the Earth’s atmosphere is there to protect us. The molecules in the atmosphere absorb the high-energy photons preventing any from reaching us on the ground. In fact, X-ray and gamma ray observatories need to be built in space because there’s no way we can see them from the ground.

Risk: Ultraviolet radiation. The Sun is bathing the Earth in ultraviolet radiation; that’s why you get a sunburn. But the ozone layer is a special region of the atmosphere that absorbs much of this radiation. Without the ozone layer we would be much more exposed here on the surface of the Earth to UV rays, leading to eye damage and greater incidence of skin cancer.

Risk: Solar flares. Violent explosions on the surface of the Sun release a huge amount of energy as flares. In addition to a blast of radiation, it often sends out a burst of plasma traveling at nearly the speed of light. The Earth’s magnetosphere protects us here on Earth from the effects of the plasma, keeping it safely away from the surface of the planet. And our atmosphere keeps the X-ray/gamma ray radiation out.

Risk: Cold temperatures. Space itself is just a few degrees above absolute zero, but our atmosphere acts like a blanket, keeping warm temperatures in. Without the atmosphere, we’d freeze almost instantly.

Risk: Vacuum. Space is airless. Without the Earth, there’d be no air to breath, and the lack of pressure damages cells and lets water evaporate out into space. Vacuum would be very, very bad.

If you’d like to hear more about cosmic rays, listen to this episode of Astronomy Cast.

References:
NASA: Danger of Solar and Cosmic Radiation in Space
NASA: Ultraviolet Waves

[/caption]
The biggest stars in the Universe are the monster red hypergiants, measuring up to 1,500 times the size of the Sun. But what are the smallest stars in the Universe?

The smallest stars around are the tiny red dwarfs. These are stars with 50% the mass of the Sun and smaller. In fact, the least massive red dwarf has 7.5% the mass of the Sun. Even at this smallest size, a star has the temperature and pressures in its core so that nuclear fusion reactions can take place.

One example of red dwarf star is the closest star to Earth, Proxima Centauri, located just 4.2 light-years away. Proxima Centauri has 12% the mass of the Sun, and it’s estimated to be just 14.5% the size of the Sun. The diameter of Proxima Centauri is about 200,000 km. Just for comparison, the diameter of Jupiter is 143,000 km, so Proxima Centauri is only a little larger than Jupiter.

But that’s not the smallest star ever discovered.

The smallest known star right now is OGLE-TR-122b, a red dwarf star that’s part of a binary stellar system. This red dwarf the smallest star to ever have its radius accurately measured; 0.12 solar radii. This works out to be 167,000 km. That’s only 20% larger than Jupiter. You might be surprised to know that OGLE-TR-122b has 100 times the mass of Jupiter, but it’s only a little larger.

And that is the smallest known star. But there are certainly smaller stars out there. The smallest theoretical mass for a star to support nuclear fusion is 0.07 or 0.08 solar masses, so smaller stars are out there.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Stars are mostly balls of hydrogen gas that came together from a nebula of gas and dust. They generate their energy through the process of fusion. This is where atoms of hydrogen are combined together to form helium atoms. And in the process, the star generates a tremendous amount of energy in the form of radiation. So, why do stars die?

This radiation starts up being trapped inside the star, and it can take more than 100,000 years to work its way out. You might not realize it, but light can emit a force when it bumps up against something. So all the light inside the star emits a pressure that opposes the force of gravity pulling all the material inward.

A star can exist in relative stability in this way for billions of years. Eventually, though, the star runs out of hydrogen fuel. At this point, a new reaction takes over, as helium atoms are fused together into heavier and heavier elements, like carbon and oxygen.

Once the helium is used up, a medium-mass star like our Sun just runs out of fuel. It can no longer sustain a fusion reaction. And without the pressure of the light ballooning it out, the star contracts down into a white dwarf – made mostly out of carbon.

A white dwarf star shines because it’s still very hot, but it slowly cools down over time. Eventually it will become cool enough that it’s invisible. And if we could wait long enough, the star would become a black dwarf star. The Universe hasn’t existed long enough for us to have any black dwarfs, but there are plenty of white dwarfs.

We have written many articles about stars here on Universe Today. Here’s an article about a hypergiant star that’s about to die.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

[/caption]
Head outside on a dark night and look up into the night sky. If you’re away from the bright city lights and it’s a clear night, you should see beautiful stars shining in the night. Just think, the light from those stars has traveled light-years through space to reach your eyes. But why do stars shine at all? Where is the light coming from?

All stars, and our own Sun is just an example, are hot balls of glowing plasma held together by their own gravity. And the gravity of a star is very intense. Stars are continuously crushing themselves inward, and the gravitational friction of this causes their interiors to heat up. A star like the Sun is a mere 5,800 Kelvin at its surface, but at its core, it can be 15 million Kelvin – now that’s hot!

The intense pressure and temperature at the core of a star allow nuclear fusion reactions to take place. This is where atoms of hydrogen are fused into atoms of helium (through several stages). This reaction releases an enormous amount of energy in the form of gamma rays. These gamma rays are trapped inside the star, and they push outward against the gravitational contraction of the star. That’s why stars hold to a certain size, and don’t continue contracting. The gamma rays jump around in the star, trying to get out. They’re absorbed by one atom, and then emitted again. This can happen many times a second, and a single photon can take 100,000 years to get from the core of the star to its surface.

When the photons have reached the surface, they’ve lost some of their energy, becoming visible light photons, and not the gamma rays they started out as. These photons leap off the surface of the Sun and head out in a straight line into space. They can travel forever if they don’t run into anything.

When you look at a star like Sirius, located about 8 light-years away, you’re seeing photons that left the surface of the star 8 years ago and traveled through space, without running into anything. Your eyeballs are the first thing those photons have encountered.

So why do stars shine? Because they have huge fusion reactors in their cores releasing a tremendous amount of energy.

We have written many articles about stars here on Universe Today. Here’s an article about an artificial star that astronomers create, and here’s an article about a star that recently shut down nuclear fusion in its core.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
University of Illinois
NASA

As you probably can guess, our Sun is an average star. Stars can be bigger than the Sun, and stars can be smaller. Let’s take a look at the size of stars.

The smallest stars out there are the tiny red dwarfs. These are stars with no more than 50% the mass of the Sun, and they can have as little as 7.5% the mass of the Sun. This is the minimum mass you need for a star to be able to support nuclear fusion in its core. Below this mass and you get the failed star brown dwarfs. One fairly well known example of a red dwarf star is Proxima Centauri; the closest star to Earth. This star has about 12% the mass of the Sun, and about 14% the size of the Sun – about 200,000 km across, which is only a little larger than Jupiter.

Our own Sun is an example of an average star. It has a diameter of 1.4 million kilometers… today. But when our Sun nears the end of its life, it will bloat up as a red giant, and grow to 300 times its original size. This will consume the orbits of the inner planets: Mercury, Venus, and yes, even Earth.

An example of a larger star than our Sun is the blue supergiant Rigel in the constellation Orion. This is a star with 17 times the mass of the Sun, which puts out 66,000 times as much energy. Rigel is estimated to be 62 times as big as the Sun.

Bigger? No problem. Let’s take a look at the red supergiant Betelgeuse, also in the constellation Orion. Betelgeuse has 20 times the mass of the Sun, and it’s nearing the end of its life; astronomers think Betelgeuse might explode as a supernova within the next 1,000 years. Betelgeuse has bloated out to more than 1,000 times the size of the Sun. This would consume the orbit of Mars and almost reach Jupiter.

But the biggest star in the Universe is thought to be the monster VY Canis Majoris. This red hypergiant star is thought to be 1,800 times the size of the Sun. This star would almost touch the orbit of Saturn if it were in our Solar System.

We have written many articles about stars here on Universe Today. Here’s an article about the biggest star in the Universe, and here’s a more detailed article about red dwarfs.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

References:
http://www.telescope.org/pparc/res8.html
http://en.wikipedia.org/wiki/Proxima_Centauri
http://www.windows2universe.org/sun/statistics.html
http://earthsky.org/brightest-stars/blue-white-rigel-is-orions-brightest-star