Would A White Dwarf Outlive A Neutron Star?

This multiwavelength composite shows the supernova remnant IC 443, also known as the Jellyfish Nebula. Fermi GeV gamma-ray emission is shown in magenta, optical wavelengths as yellow, and infrared data from NASA's Wide-field Infrared Survey Explorer (WISE) mission is shown as blue (3.4 microns), cyan (4.6 microns), green (12 microns) and red (22 microns). Image credit: NASA/DOE/Fermi LAT Collaboration, Tom Bash and John Fox/Adam Block/NOAO/AURA/NSF, JPL-Caltech/UCLA

This multiwavelength composite shows the supernova remnant IC 443, also known as the Jellyfish Nebula. Fermi GeV gamma-ray emission is shown in magenta, optical wavelengths as yellow, and infrared data from NASA's Wide-field Infrared Survey Explorer (WISE) mission is shown as blue (3.4 microns), cyan (4.6 microns), green (12 microns) and red (22 microns). Image credit: NASA/DOE/Fermi LAT Collaboration, Tom Bash and John Fox/Adam Block/NOAO/AURA/NSF, JPL-Caltech/UCLA

Originally posted at Forbes!

This is an interesting question, because most of the time when we talk about the lifetime of a star, we mean the part before the creation of a white dwarf or a neutron star.  Typically, a star has a “lifetime” when it is burning hydrogen into helium, gradually consuming its central reservoir of hydrogen, producing light as it does so.

It’s using this kind of definition for “lifetime” that we say our home star Sol is middle aged. Our Sun is a little over 4.5 billion years old, about halfway through its estimated ~9 billion years of hydrogen burning. Once hydrogen burning is complete, our Sun, along with all other stars of its mass, will go through an elaborate end-of-life transformation, which involves shedding most of the outer layers of the star. In the case of the Sun, we’ll produce a planetary nebula out of the discarded outer surface of the star, leaving only the hot nucleus of the star behind as a white dwarf.

This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. A version of the image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt. Image credit: ESA/Hubble & NASA; acknowledgement: J. Schmidt (geckzilla.com)

This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. A version of the image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt. Image credit: ESA/Hubble & NASA; acknowledgement: J. Schmidt (geckzilla.com)

If the star were a bit larger, it would explode as a supernova instead of creating a planetary nebula and white dwarf. The star has to sit within a very specific range of masses in order to create a neutron star - too large, and the gravitational weight of the star can compress the core of the star down into a black hole, instead of stopping at a neutron star. If the star is too light, it won’t supernova at all, and you’re left with a white dwarf instead of a neutron star.

Whether you have a white dwarf or a neutron star, both are considered stellar remnants. They’re what remains of a star, after fusion has stopped in the core of the star, and gravity has dealt with the unstable star. Neither the white dwarf or the neutron star will be generating any new heat in their cores, or will be able to do much else, beyond sit there, without some kind of external influence.

A dying star is throwing a cosmic tantrum in this combined image from NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX). In death, the star's dusty outer layers are unraveling into space, glowing from the intense ultraviolet radiation being pumped out by the hot stellar core. This object, called the Helix nebula, lies 650 light-years away, in the constellation of Aquarius. Image credit: NASA/JPL-Caltech

A dying star is throwing a cosmic tantrum in this combined image from NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX). In death, the star's dusty outer layers are unraveling into space, glowing from the intense ultraviolet radiation being pumped out by the hot stellar core. This object, called the Helix nebula, lies 650 light-years away, in the constellation of Aquarius. Image credit: NASA/JPL-Caltech

A white dwarf, on its own, will sit in place and glow, gradually losing energy to the deep dark of space, until the white dwarf has lost enough heat to match the background temperature of the universe, at about 3 degrees Kelvin. If the white dwarf could achieve this, we would call the resulting object a black dwarf, but so far, none have ever been observed. This lack of black dwarfs isn't a surprise - the amount of time a white dwarf is predicted to need in order to get to black dwarf status is longer than the Universe has been around. There quite literally hasn’t been enough time in the entire universe for a white dwarf to cool.

Neutron stars are also high temperature objects, and all they can do is cool, but they come equipped with frankly astoundingly strong magnetic fields, and so their cooling process is a little more involved. We spot many neutron stars in the night skies by watching them beam X-rays away from themselves. Combined with the extremely rapid rotation that a lot of neutron stars have, neutron stars are often picked out by watching them pulse in brightness, like a high energy radiation lighthouse.

Without some kind of external factor coming into play, neither a white dwarf or a neutron star will cease to exist in our universe, so in that regard a neutron star and a white dwarf are tied for longevity. There are external factors you could invoke to dispose of a white dwarf or a neutron star - these usually come by way of a second star, which donates mass to the white dwarf or to the neutron star. If the white dwarf gains enough material from devouring its neighbor star, it can explode in a supernova, detonating itself into oblivion. A neutron star can also gather enough mass to itself so that it collapses into a black hole, imploding inwards on itself. However, there's no reason for the white dwarf to always do this faster than a neutron star, because the key factor here is how quickly the star can steal material from its companion. The speed of the theft depends on the orbit of the companion, not whether or not you have a white dwarf instead of a neutron star.

Without such a stellar donor, both of these stellar remnants will simply hang out in the Universe, slowly losing their heat, until they have matched the background temperature of the Universe.

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Why Does The Moon Rise Later Each Day?

Why does the Moon rise 30 to 70 mins later each day than the previous day?
The Galileo spacecraft took this 1992 shot showing the Moon in orbit about Earth. With only a third of the brightness of Earth, the Moon has been digitally enhanced to improve visibility. Image credit: NASA

The Galileo spacecraft took this 1992 shot showing the Moon in orbit about Earth. With only a third of the brightness of Earth, the Moon has been digitally enhanced to improve visibility. Image credit: NASA

Originally posted on Forbes!

The Moon does indeed rise on average 50 minutes later each day in our skies, which may come as a surprisingly large daily change, particularly if you’re used to the much more gradual changes of sunrise and sunset times. To explore why the Moon’s arrival in the sky changes so much, let’s look into the Moon’s orbit around our Earth.

The Moon orbits our home planet once every 28 days or so, and as it orbits our planet, the angle between the Sun, the Earth and the Moon changes. If we think about looking down on the solar system from above, the Earth spins like a top below us. On a slower cadence, the Moon drifts in a wide ellipse around the Earth. The Sun itself is a distant, stable source of light. When the Moon is to the left or the right of the Earth (if we place the Sun at a distant 12 o'clock), an observer on the portion of the Earth facing the Moon would see the sunward half of the Moon illuminated by daylight. The other half of the Moon, which falls in shadow, would appear dark. This gives us the half-moon phase.

This is an image of Earth and the moon, acquired on October 3, 2007, by the HiRISE camera on NASA's Mars Reconnaissance Orbiter. At the time the image was taken, Earth was 142 million kilometers (88 million miles) from Mars, giving the HiRISE image a scale of 142 kilometers (88 miles) per pixel. The moon image is brightened relative to Earth for this composite. Image credit: NASA/JPL-Caltech/University of Arizona

This is an image of Earth and the moon, acquired on October 3, 2007, by the HiRISE camera on NASA's Mars Reconnaissance Orbiter. At the time the image was taken, Earth was 142 million kilometers (88 million miles) from Mars, giving the HiRISE image a scale of 142 kilometers (88 miles) per pixel. The moon image is brightened relative to Earth for this composite. Image credit: NASA/JPL-Caltech/University of Arizona

If the Moon is between the Sun and the Earth, an earthbound observer is stuck looking at the shadowed half of the Moon - the daylit side is facing away from us. And when the Moon is on the other side of the Earth, so that the Earth is between the Sun and Moon, the half of the Moon which is illuminated is also the half of the Moon which faces the surface of the Earth, giving us a full moon. So it is this changing alignment between the Earth, where we are looking from, the Sun, which provides the light to the surface of the Moon, and the Moon’s position itself, which gives us these changing phases of the Moon. Along with the Moon’s orbit, these phases cycle every 28 days.

This physical motion of the Moon around around the Earth every 28 days is half the puzzle of the changing Moon rising times. The other half is that the Earth is spinning much faster than the Moon is orbiting. The Earth rotates once every 24 hours, so if you’re standing motionless on the surface of the Earth, looking up, any object in space which isn’t also orbiting our planet every 24 hours will appear to traverse the skies. This is true of the Sun, which crosses the skies every day (by definition), and is also true of the Moon.

Taken June 21, 2016 by Commander Jeff Williams of NASA during Expedition 48 on the International Space Station. Image credit: NASA

Taken June 21, 2016 by Commander Jeff Williams of NASA during Expedition 48 on the International Space Station. Image credit: NASA

Moonrise happens when the Earth has rotated enough on its own axis that the Moon has appeared in your personal sky - your horizon has caught up with the Moon. When exactly that happens is a combination of the Earth’s rapid rotation, and the Moon’s continual motion through the skies. The Moon has to make it through an entire loop of the Earth in about 28 days, which means it’s moving by about 13 degrees in the sky every day. The Earth, meanwhile, rotates through 15 degrees every hour, in order to rotate 360 degrees every 24 hours.

The Moon is continually moving on ahead in its orbit while the Earth rotates. So 24 hours later, the Earth has rotated back around to the same place it was the night before, but the Moon has gone on ahead. Think of it like the second hand on an analog watch; it’s going around the face of the clock much faster than the minute hand, but each time the second hand goes around, the minute hand has moved, and so it takes an extra second to line back up with the minute hand. Because the Moon has moved 13 degrees or so since its last moonrise, it’s going to take another hour or so for the Earth to catch back up to the Moon’s new location, delaying the Moon's rising above your horizons by ~50 minutes each day.

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Why Are Earth And Venus Called Twins?

Why are Earth and Venus called twins?
This global view of the surface of Venus is centered at 180 degrees east longitude. Magellan synthetic aperture radar mosaics from the first cycle of Magellan mapping are mapped onto a computer-simulated globe to create this image. Image credit: NASA/JPL

This global view of the surface of Venus is centered at 180 degrees east longitude. Magellan synthetic aperture radar mosaics from the first cycle of Magellan mapping are mapped onto a computer-simulated globe to create this image. Image credit: NASA/JPL

Originally posted on Forbes!

The Earth and Venus do often get called planetary twins, and this is largely because they are very close to being the same mass. Both the Earth and Venus are rocky planets, which means that they're effectively the same density (which can not be said of the Earth and, say, Neptune), and so they are also very nearly the same physical size. They also both have significant atmospheres surrounding their surfaces. However, their evolutionary pathways since the time of the early solar system have taken both planets down dramatically different tracks, in spite of all their similarities.

The Earth clocks in at a very respectable 5.97 x 10^24 kilograms of mass. Venus sits at 81.5% of this, at 4.867 × 10^24 kg. Venus' radius is only a few hundred kilometers smaller than the Earth's - ~6050 km instead of ~6370, a difference of only about 5%. If we were only looking at their size, rock type, and mass, you might expect that Venus and Earth should be pretty similar, and this is precisely where the “twin” styling comes from.

Computer generated surface view of Eistla Regio (from the northeast). Vertical scale has been exaggerated by a factor of 22.5. Image credit: NASA

Computer generated surface view of Eistla Regio (from the northeast). Vertical scale has been exaggerated by a factor of 22.5. Image credit: NASA

It doesn’t take much further examination to reveal that Venus is a very different place to our home, and much stranger than we originally thought. Unlike most of the other planets in the solar system, which rotate counterclockwise (as viewed looking down from the North Pole), Venus rotates clockwise. (You could equally fairly say that Venus is simply upside down.) It also does so extremely slowly -- Venus is the slowest rotator in the entire solar system. Each ‘day’ lasts for approximately 117 Earth days - in other words, the Earth will have spun 117 times in the time it took Venus to rotate once around its own axis. Considering that Venus’ year is shorter than the Earth’s (it is closer to the Sun, after all), Venus’ year takes about two Venusian days.

This picture of Venus was taken by the Galileo spacecraft's Solid State Imaging System on February 14, 1990, at a range of almost 1.7 million miles from the planet. A highpass spatial filter has been applied in order to emphasize the smaller scale cloud features, and the rendition has been colorized to a bluish hue in order to emphasize the subtle contrasts in the cloud markings and to indicate that it was taken through a violet filter. Image credit: NASA/JPL

This picture of Venus was taken by the Galileo spacecraft's Solid State Imaging System on February 14, 1990, at a range of almost 1.7 million miles from the planet. A highpass spatial filter has been applied in order to emphasize the smaller scale cloud features, and the rendition has been colorized to a bluish hue in order to emphasize the subtle contrasts in the cloud markings and to indicate that it was taken through a violet filter. Image credit: NASA/JPL

Venus is shrouded with a thick, dense atmosphere, far thicker than our own. On Earth, our atmosphere is thick enough to produce a significant amount of pressure on the surface, but our planet is not totally cloud-covered. Our Earth-monitoring satellites are regularly able to see the ground from space, without the interference of the clouds. There’s no such break in the clouds on Venus. Venus is permanently clouded over, and its atmosphere is so thick that the surface pressure on Venus is 92 times the pressure here on Earth. An unshielded human would fare very badly in this environment.

The atmosphere of Venus is a Level 99 Expert at trapping heat. It’s almost entirely carbon dioxide, which, as we know from our own planet, is a greenhouse gas. With such a high concentration of heat-trapping gas in the atmosphere, the surface temperature of Venus sits at a temperature hotter than 850 degrees Fahrenheit.

Color version of the left half of a Venera 13 image of the surface of Venus. Image credit: NASA

Color version of the left half of a Venera 13 image of the surface of Venus. Image credit: NASA

The crushing pressure and searing heat makes Venus a particularly difficult place to explore. The electronics in most robotic missions generally do not handle these kinds of temperatures very well; to function at all, they need to be reinforced against the pressure. This is the space equivalent of sending your delicate machinery to about 900 meters below the ocean, but where the ocean is 50% of the way to being as hot as lava. We’ve sent relatively few landers to Venus, and the most recent of them (Vega 2) dates back to 1985, operating for just under an hour before succumbing to the heat.

A volcano named Sapas Mons dominates this computer-generated view of the surface of Venus. Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground to the base of the mountain, which measures 248 miles across by 0.9 miles high. The image was produced by the Solar System Visualization project and the Magellan Science team at the JPL Multimission Image Processing Laboratory. Image credit: NASA/JPL

A volcano named Sapas Mons dominates this computer-generated view of the surface of Venus. Lava flows extend for hundreds of kilometers across the fractured plains shown in the foreground to the base of the mountain, which measures 248 miles across by 0.9 miles high. The image was produced by the Solar System Visualization project and the Magellan Science team at the JPL Multimission Image Processing Laboratory. Image credit: NASA/JPL

There is another similarity between the Earth and Venus, though not one that makes Venus a more hospitable place to go visit: both planets have volcanoes. Because Venus is so hot and the pressures are so great, the volcanoes on Venus’ surface aren’t quite as vertically imposing as they can be on Earth. These Venusian volcanoes are much larger, in that they cover a much greater area, but they're extremely flat. Sapas Mons, in the image above, covers 250 miles from edge to edge, but only rises to 4752 feet. (The image above has had its vertical scale exaggerated by a factor of twenty two). This volcano covers roughly the same area as the entire state of New York. Venusian volcanoes dwarf most of their Earthly counterparts in area. I'm biased, but I think you'll agree that Earth has it grandly outdone for scenery.

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Why Aren't The Van Allen Belts A Barrier To Spaceflight?

I follow all kinds of information about space and the stars. My brother has only recently started paying attention to these issues, but has been reading some naysayer websites. Because of this, he says he has doubts about the ‘truth’ of the space shuttle, the flight to the moon and other missions, as some claim that they would be impossible because of the heated layers of atmosphere around the earth, which would destroy them—the Van Allen belts. I know that heat shields are used, and am assuming that the rarefied atmosphere might not conduct heat as well. But what is the real reason why these flights are possible and are not eliminated by the heat of the Van Allen belts or other layers?
In a very unique setting over Earth's colorful horizon, the silhouette of the space shuttle Endeavour is featured in this photo by an Expedition 22 crew member on board the International Space Station, as the shuttle approached for its docking on Feb. 9 during the STS-130 mission. Image credit: NASA

In a very unique setting over Earth's colorful horizon, the silhouette of the space shuttle Endeavour is featured in this photo by an Expedition 22 crew member on board the International Space Station, as the shuttle approached for its docking on Feb. 9 during the STS-130 mission. Image credit: NASA

Originally posted on Forbes!

So there are two questions mixed up in here - the first is about traversing the atmosphere without burning up, and the second about traversing the Van Allen belts.

It’s true that re-entering the atmosphere from space is a delicate business, and there are only a few safe paths to do so. The atmosphere, as easily as we move through it on the surface of the Earth, can pose a significant barrier to fast-moving objects. Air resistance is a major factor in designing everything from cars to parachutes to space shuttles. If you’ve ever been out in high winds, you’ve felt the kind of barrier wind can produce to your own motion, and how much force it takes to move in resistance to it.

Objects which encounter our atmosphere from space are generally travelling much faster than any winds we’d encounter during a storm here on Earth (thank goodness), and so the air resistance they hit is significant; the atmosphere, if hit directly, is almost as solid a barrier as encountering rock. Crew-carrying spacecraft will never plunge straight down into the atmosphere, but encounter it at a shallow angle, which allows the craft to encounter the atmosphere’s resistance less abruptly.

This computer-generated art depicts Orion's heat shield protecting the crew module as it enters the Earth's atmosphere. Image credit: NASA

This computer-generated art depicts Orion's heat shield protecting the crew module as it enters the Earth's atmosphere. Image credit: NASA

So what does this atmospheric resistance do? It slows down the spacecraft, by absorbing some of the spacecraft’s energy. That energy heats up the atmosphere immediately around the craft, encasing the craft in a superheated plasma for part of its descent, until much of the forward motion of the craft has been lost. By approaching the atmosphere at an angle, this process takes a longer time, and the craft can be safely slowed. If we tried to drop straight down into the atmosphere, the craft would not be able to slow down as much, and the sudden increase in pressure from the atmosphere would put so much stress on the craft that it might break. If you have humans in the craft, this is not a good idea. If, on the other hand, you’re just trying to get a satellite out of orbit, you can drop them into the atmosphere at a steeper angle, as they don’t need to be functional when they plunge into the Pacific Ocean. (That’s usually where we put them.)

So yes, there’s a heating problem when you re-enter the atmosphere, but the atmosphere itself isn’t heated any more than ambient air temperature. It's only the air surrounding the craft which heats, and only because there's a spacecraft barreling through. The upper atmosphere is actually quite cold, so there’s no intrinsic heated barrier to traverse. We don’t have the same heating problem when launching a spacecraft, after all. This heating is simply atmospheric drag, though this is dangerous enough - the loss of heat tiles protecting the wings of the space shuttle was what led to the loss of the Shuttle Columbia.

NASA's Van Allen Probes orbit through two giant radiation belts that surround Earth. Their observations help improve computer simulations of events in the belts that can affect technology in space. Image credit: JHU/APL, NASA

NASA's Van Allen Probes orbit through two giant radiation belts that surround Earth. Their observations help improve computer simulations of events in the belts that can affect technology in space. Image credit: JHU/APL, NASA

The Van Allen belts, on the other hand, are not actually part of our atmosphere. They’re well beyond it, extending hundreds of miles outwards into space. There are two, both donut-shaped rings surrounding our planet, and are a consequence of our planet’s magnetic field. The Space Shuttle typically orbited at a height of 190 miles to 330 miles above the surface, and the International Space Station orbits at a height of somewhere between 205 and 270 miles above the surface of the Earth.

The innermost Van Allen belt sits somewhere between 400 to 6,000 miles above the surface of our planet. Even if the innermost belt is at its closest, the ISS (and the space shuttle in its day) are more than 100 miles away from the Van Allen Belts. For near-Earth missions, the Van Allen belts are not a hazard to spacefarers.

It was, however, a hazard for the Apollo missions. The Van Allen belts are not a physical barrier to spacecraft, and so, in principle, we could have sent the Apollo spacecraft through the belts. It would not have been a good idea. The Van Allen belts are a kind of trap for charged particles like protons and electrons. They’re held in place by the magnetic field of the Earth, and so they trace the shape of the magnetic field itself. The problem with the Van Allen belts lies not in them being impassable, but in the charged particles they contain.

In this 1966 photo, a plasma thruster at NASA's Lewis Research Center simulates Van Allen Belts, rings of radiation around the Earth. The Cleveland, Ohio, center is now John H. Glenn Research Center. Image credit: NASA

In this 1966 photo, a plasma thruster at NASA's Lewis Research Center simulates Van Allen Belts, rings of radiation around the Earth. The Cleveland, Ohio, center is now John H. Glenn Research Center. Image credit: NASA

Charged particles are damaging to human bodies, but the amount of damage done can range from none to lethal, depending on the energy those particles deposit, the density of those particles, and the length of time you spend being exposed to them.

In the case of the Apollo missions, the solution was to minimize the second two factors. We can’t control the energy of those particles, though they can be large. The density of the Van Allen belts is well known (from sending uncrewed probes through them), and there are hotspots you can definitely avoid. In particular, the innermost belt is a rather tightly defined region, and it was possible to stay out of it for the trip to the Moon. The second belt is much larger, and harder to avoid, but there are still denser regions to avoid. For the Apollo trips, we wanted to send the astronauts through a sparse region of the belts, and to try and get through them quickly. This was necessary in any case; the crafts had to make it to the Moon in a reasonable amount of time, and the shorter the trip, the less exposure to all sorts of radiation the astronauts would get.

An artist's depiction with cutaway section of the two giant donuts of radiation, called the Van Allen Belts, that surround Earth. Image credit: NASA/Goddard Space Flight Center/Scientific Visualization Studio

An artist's depiction with cutaway section of the two giant donuts of radiation, called the Van Allen Belts, that surround Earth. Image credit: NASA/Goddard Space Flight Center/Scientific Visualization Studio

In the end, it seemed that these tactics worked; the on-board dose counters for the Apollo missions registered average radiation doses to the skin of the astronauts of 0.38 rad. This is about the same radiation dose as getting two CT scans of your head, or half the dose of a single chest CT scan; not too bad, though not something you should do every week.

Your brother is right that both the atmosphere and the Van Allen belts can be dangers to space exploration, but with careful observations, orbital maneuvering, and inventiveness, we’ve navigated our way beyond them many times. Hopefully, we'll continue to do so in the future many times more.

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