Haven't The Stars In The Oldest Galaxies Died By Now?

If it has taken 13 or so billion light years for the light from the most distant observable stars to reach Earth, is it not probable that those stars no longer exist but have “gone nova”?

It’s not just probable; it’s a certainty! Most of the brightest stars have lifetimes that only keep them going for a few hundred million...

Read the full article on Forbes!

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If Time Doesn't Exist For Photons, How Does Anything Happen To It?

Images showing the expansion of the light echo of V838 Monocerotis. Image credit: NASA, ESA, H.E. Bond (STScI) and The Hubble Heritage Team (STScI/AURA)

Images showing the expansion of the light echo of V838 Monocerotis. Image credit: NASA, ESA, H.E. Bond (STScI) and The Hubble Heritage Team (STScI/AURA)

Originally posted at Forbes!

The concept of photons running with stopped clocks is something that is pulled straight out of relativity; the faster you’re moving, the slower your onboard clocks are moving, and the closer to the speed of light you’re operating, the more sluggish they get. Once you reach the speed of light, your clock runs infinitely slow - for practical purposes, we can say that time doesn't flow for the photon. As with all things relativity, this isn’t an absolute statement- light still has a finite speed, and we can observe light taking fixed amounts of time to traverse large distances.

When light goes zipping around our Universe, it is physically moving through space at a speed of 186,000 miles every second.  But if you could affix a clock to it, an observer that’s not moving at the speed of light would not see the clock moving forwards the way their own clocks do. A hypothetical person moving at the speed of light wouldn’t notice anything weird with their clock, but what they might notice is that the Universe is full of things to smash into.

This artist's impression shows how photons from the early universe are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image credit: ESA

This artist's impression shows how photons from the early universe are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image credit: ESA

No matter how fast you’re going, if there’s something in front of you, and you can’t dodge it, you will hit it. This is as true for humans as it is for light, and light is even less capable of dodging an oncoming object than we humans are.  Light always travels in locally straight lines - the only way to bend light is to make a curve in the shape of space. A photon will then follow that curve, but there’s no onboard navigation.

Photons are effectively stuck playing the world’s most obnoxious game of bumper cars, continually bouncing from impact to impact. From our non-speedy perspective, the clocks on photons do not tick forward between impacts, so if the photon has the good fortune to get re-emitted by whatever it ran into, it will, from our viewpoint, instantaneously smash directly into something else without its onboard clock ticking onwards at all.

The photon may not get re-emitted by whatever it ran into, (this is one way to get rid of a photon). The energy of whatever it hit will increase, so the energy isn’t lost. However, if it hits something particularly cold, the object won’t be radiating much, and the photon’s energy will be a convenient donation.  More commonly, after some amount of time, a new photon will be produced, at a different energy level, carrying energy away from whatever the photon punched itself into earlier. That new photon has an equally short apparent flight until it smashes into something else.

It’s not the most glamorous of paths through the Universe, but a continual ricocheting from solid matter to solid matter is how photons in our Universe go about it.

The bright cloud is a reflection nebula known as [B77] 63, a cloud of interstellar gas that is reflecting light from the stars embedded within it. There are actually a number of bright stars within [B77] 63. Image credit: ESA

The bright cloud is a reflection nebula known as [B77] 63, a cloud of interstellar gas that is reflecting light from the stars embedded within it. There are actually a number of bright stars within [B77] 63. Image credit: ESA

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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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