Can A Star Ever Turn Its Spin Backwards?

Can a star reverse its rotational direction during some time in their life, and if so, how would it affect any planets around it?
This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

Originally posted on Forbes! 

The stars in the night sky all have their preferred direction of rotation, which depends directly on the exact way that the cloud of gas and dust that the star formed out of collapsed. If there was slightly more random motion in a clockwise or a counterclockwise direction, as the cloud of gas collapsed, the star would have magnified that hint of rotation, spinning up the same way that a figure-skater does, pulling in their arms and legs.

Once spun up this way, another piece of fundamental physics comes into play - inertia. Inertia tells us that objects in motion tend to stay in motion unless there’s something else that’s causing that object to slow down. On Earth, that something else can come in many forms - we have the mass of the Earth, whose gravity will pull objects down towards the surface, an atmosphere to move through, which will slow objects moving through it, or quite simply mountains and buildings which objects can bounce off of and away from.

If you’re in space, these sorts of Earthly obstacles which can serve to stop a moving object aren’t around. There are many fewer things which could serve to stop an object’s motion - without a thick atmosphere to move through, the planets and our spacecraft continue at their current speeds without any impediments. The most common method of slowing down (or speeding up) an object in space is by traveling near another large object (ones that are of a similar mass to yourself are the most effective) and letting the force of gravity alter your path.

These requirements for an external force hold both for motion as we normally think about it (a forward or sideways motion) and for spin. So if we think about spinning a bicycle tire, that wheel will continue to spin until the forces of friction in the axle (primarily) will slow it down, or until you clamp down on the brakes. An object which is spinning in space has no friction-containing axle around which to spin, and so if it’s isolated, without any external objects which can act as a braking force, that object should continue to spin as it is, ad infinitum. This is basically the situation that stars find themselves in. Stars do not reverse their spins as a standard part of their lifetimes.

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

A reversal in the rotation of a star is extremely difficult to accomplish without something external to the star punching it backwards in the other direction, to slow down each rotating particle that makes up the star. If you could, using magic, reverse the rotation of our own star, without changing anything else, the planets surrounding our Sun wouldn’t be influenced at all - the orbits of the planets are determined by the gravitational pull of the Sun, which hasn’t changed if we haven’t changed its mass, plus the planets’ own velocities.

What sort of objects could act as a brake in space? The is easiest is always a good old large-scale collision. This is how we think the Earth formed the Moon, how Uranus got tipped onto its side, and Venus got tipped completely upside down. So if we wanted to reverse the spin of the Sun, we’d have to hit it with something pretty catastrophic in order to both stop the rotation of the gas & plasma which currently makes up the Sun, and reverse the direction of the spin. Any impact on that scale would definitely impact the planets. If it was another star that hit our own, we’d have gravitational and temperature-related chaos even before the impact, setting aside whatever cataclysm of energy would be unleashed during the collision.

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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”?

Originally posted on Forbes!

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...

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 years. Even if the light caught in our detectors was generated in that star’s earliest days, by the time that light reached us, 13 billion years later, that star must be long gone. Our own Sun, which has a comparatively lengthy 8 billion year lifespan, wouldn’t have survived for the length of time it takes for the light from the most distant known galaxies to reach us.

What does that mean for the galaxy as we see it? For starters, it means that its population of stars has changed almost entirely since that light began its travels. The brightest stars have all died, exploding out to recycle their gas into their stellar neighborhood, and possibly triggering the formation of a new round of stars. In 13 billion years, this may happen many, many times. But the stellar recycling act isn’t perfect. For every large, extremely bright star that’s formed, we typically expect a number of smaller, fainter stars to also form. These smaller stars (like our Sun) live longer and don’t explode as violently (or at all) at the end of their lifetimes. Stars which are even smaller and redder than our own will live even longer, and may even persist for the entire length of the trip the light took to reach us. This means that over time, the galaxy will build up a reservoir of faint, reddish stars, which limits the amount of gas present in the galaxy available to make new stars. This sequence of events is one of the suggestions for how we end up with the giant elliptical galaxies in the nearby universe -- their population of stars is mostly red, and they seem to have very little gas.

A lone source shines out brightly from the dark expanse of deep space, glowing softly against a picturesque backdrop of distant stars and colorful galaxies. This scene shows PGC 83677, a lenticular galaxy — a galaxy type that sits between the more familiar elliptical and spiral varieties in the Hubble sequence. Image credit: ESA/Hubble & NASA | Acknowledgements: Judy Schmidt (Geckzilla)

A lone source shines out brightly from the dark expanse of deep space, glowing softly against a picturesque backdrop of distant stars and colorful galaxies. This scene shows PGC 83677, a lenticular galaxy — a galaxy type that sits between the more familiar elliptical and spiral varieties in the Hubble sequence. Image credit: ESA/Hubble & NASA | Acknowledgements: Judy Schmidt (Geckzilla)

But it’s not the only pathway open to that distant galaxy -- it’s also possible to refill the galaxy with gas, allowing the galaxy to continue forming the brightest, bluest, shortest-lived stars for a longer period of time. Depending on whether or not the galaxy finds itself surrounded by smaller, gas-filled galaxies, or on its own in a more lonely part of the Universe, that distant galaxy’s course will change again. If we could sit and watch that distant galaxy’s evolution for a few billion more years, we would be able to say for sure which pathway that galaxy was sent down. I don’t know about you, but I certainly don’t have a hundred million years to wait.

So without millions or billions of years to wait for updates from that galaxy, we’re a bit stuck. We effectively have a snapshot of this earliest galaxy as it was, and no ability to check what it did later, or how the galaxies around it changed with time. What we do have is another snapshot of the Universe later, where a different set of galaxies exist, with the same inability to watch where they go forward in their path through their own lives. Trying to piece together which galaxies in the distant universe might evolve into the galaxies we see at more recent times, at less extreme distances, is one of the fundamental puzzles of observational astronomy.

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Do Stars Within A Galaxy Touch One Another?

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Why are there no green stars?

Why are there no green stars in space?
The active lava lake, Halema'uma'u crater, in Kilauea, Hawaii. Cracks in the cooler surface lava glow yellow from the heat of the molten rock. Image credit: Ivan Vtorov, CC-3.0 A-SA

The active lava lake, Halema'uma'u crater, in Kilauea, Hawaii. Cracks in the cooler surface lava glow yellow from the heat of the molten rock. Image credit: Ivan Vtorov, CC-3.0 A-SA

Originally posted at Forbes!

If you think about the sheer number of other stars out there, and the range of possible colors our eyes can detect, it seems like there should be green stars, doesn’t it? After all, there’s very clearly a green band in a rainbow. But a rainbow is just a demonstration of all the independent colors that make up the white light we get from our sun, and the sun itself is glowing to produce that light.

Takakkaw Falls, Yoho National Park, British Columbia, Canada. Image credit: Michael Rogers, CC BY-SA 3.0

Takakkaw Falls, Yoho National Park, British Columbia, Canada. Image credit: Michael Rogers, CC BY-SA 3.0

All stars glow in a particular color depending on how hot they are, but they’re not glowing only in that color in the same way that we see the green light in the rainbow only because it’s been separated from the other colors. Each star glows in a range of colors (corresponding to a range of frequencies of light), but they will produce some frequencies more than others. It turns out that we can predict the exact range of colors produced by a star, along with the frequency it produces the most. This peak frequency is determined entirely by the temperature of the star. We can do this calculation so easily because stars are well described as a black-body -- any object which reflects no light, and glows according to its own internal heat falls into this class of objects.

There are a couple of black-body objects on Earth we might be familiar with, and they’re all things we should avoid touching. Molten iron, for instance (and in fact any forged metal), will glow because of the temperature to which it has been heated in the forge. Both red-hot and white-hot metal will burn you badly, but the brighter white that metal is, the hotter it is. We actually use this property for science on Earth; for example, if you’re studying volcanoes. If you’d like to know how hot the lava is without sticking a thermometer in it by hand (potentially dangerous), you can find out the temperature by checking out the color of the uncooled lava. The closer to yellow-white that lava is, the hotter it is.

Metal, after being heated in a forge, glows bright yellow, cooling to a darker orange-red. Image credit: Alex Lines, CC 2.0 A-SA

Metal, after being heated in a forge, glows bright yellow, cooling to a darker orange-red. Image credit: Alex Lines, CC 2.0 A-SA

But this doesn’t explain the lack of green stars. We can certainly make things burn green - we’ve managed to come up with green fireworks, for instance, but this burning is not the same as heating something until it glows green. Green fireworks are usually that color because certain salts (usually copper chloride or barium chloride) have been mixed in with the gunpowder. Heating those salts makes them glow at certain frequencies, just like a fluorescent light bulb. Critically, this isn’t a heat-based process

Green fireworks explode in the night sky; the green due to the inclusion of barium salts. Image credit: Jerry Daykin, CC-2.0-A-SA

Green fireworks explode in the night sky; the green due to the inclusion of barium salts. Image credit: Jerry Daykin, CC-2.0-A-SA

I said earlier that based 100% on the temperature of the star, I can tell you the peak frequency of light that star produces. By that logic, it’s entirely possible for a star to have a most-produced color that is green. And in fact this is true; and technically this could be called a green star. However, if you were to go look at that star, or any other black body object, heat its way to a peak frequency of green, what you would see would be disappointingly un-green. As you heat from the lowest temperatures, you go from a dark red, to orange, to yellow, to white. From white, if you continue heating, you can get to blue, but blue black-body objects tend to be extremely hot, with peak frequencies in the ultraviolet, and what we’re seeing as blue is the cooler tail extending down into the blue end of the visible spectrum.

We’ve totally skipped green! We replaced it with white. Because green is close to the center of the visible range, even if a star is producing a lot of green light, it’s also producing a lot of yellow and blue light, and the mixture appears white to our eyes. If the peak of the light production is off to one side or another, we can spot the shift towards orange or blue, but with green, we see a near perfect white light.

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What are all of the stars called?

Several Geminid meteors seen from the Observatorio del Teide (IAC), Tenerife, Spain, in the morning of 14 June 2013, at about 05:30 GMT. The telescope in the foreground is ESA’s Optical Ground Station. Above is the Orion constellation; the brightest spot at the top of the image is Jupiter. Image credit:  StarryEarth via Flickr  is CC licensed  CC BY-NC 2.0

Several Geminid meteors seen from the Observatorio del Teide (IAC), Tenerife, Spain, in the morning of 14 June 2013, at about 05:30 GMT. The telescope in the foreground is ESA’s Optical Ground Station. Above is the Orion constellation; the brightest spot at the top of the image is Jupiter. Image credit: StarryEarth via Flickr is CC licensed CC BY-NC 2.0

Originally posted at Forbes!

There are actually only a handful of stars in the night sky with proper names, and they’re usually the brightest stars in the night sky. We know where to find Sirius, the brightest star in the sky (near Orion), and Betelgeuse, a supergiant star in the shoulder of Orion. If you dig a bit deeper, you will find a number of other stars with Arabic names, which date back to one of the earliest catalogues of stars, compiled by Ptolemy, and later translated into Arabic; the Arabic names were then adopted by Western star catalogues. As an example, still within Orion, the stars in Orion’s belt are named Alnilam, Alnitak, and Mintaka.

 

Unfortunately, many of the transcribers in the middle ages, being unfamiliar with Arabic words, were not always particularly consistent with their spelling of the Arabic names, so many of these stars (as they have been adopted into European usage) either have more than one spelling of their name, or have been given a name which is a loose interpretation of the Arabic spelling. Many of these names are simply a description of their location within a constellation – Mintaka and Alnitak both have names which indicate that they are in the belt, and could have been written al-Mantaqa and an-Nitāq.

Very rarely, you might find a star which is named after a particular person – Barnard’s star, for instance, which was found by E.E. Barnard. Barnard’s star is interesting because it was found to be moving very rapidly through our sky, meaning that it must be relatively close by. This sort of nomenclature tends to stick only for very unusual stars, where there isn’t already a better naming system in place.

An early attempt to make a more comprehensive catalogue of stars was the Bayer Designation system, which arose in the 16th century from Johann Bayer, and was intended to classify all the stars within a certain constellation by its brightness. Each star was given a greek letter (alpha being assigned to the brightest, beta to the second brightest, etc.) and then a modified version of the constellation name. So the brightest star in Cepheus, for instance, became Alpha Cephei, and the third brightest is Gamma Cephei. This particular catalogue has its own set of flaws, including that the brightness calibrations were not particularly precise, so the assigned order is not always exactly correct.

However, the vast majority of all stars are identified by an alphanumeric designator invented by the survey which observed them. Modern surveys involve so many objects that naming each object individually would be a huge undertaking, so there is usually an automated ‘name’ generated for each object, which can usually be based on some combination of survey name, order in which the object was observed, position on the sky, and an informational flag (such as, do we think this object is a star or a galaxy), and might wind up looking like this, which is taken from the SDSS: SDSS J113501.52+002536.5

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