galaxy Archives

June 4, 2015February 27, 2017 by Fraser Cain

How Do Galaxies Die?

Everything eventually dies, even galaxies. So how does that happen? Time to come to grips with our galactic mortality. Not as puny flesh beings, or as a speck of rock, or even the relatively unassuming ball of plasma we orbit.

Today we’re going to ponder the lifespan of the galaxy we inhabit, the Milky Way. If we look at a galaxy as a collection of stars, some are like our Sun, and others aren’t.

The Sun consumes fuel, converting hydrogen into helium through fusion. It’s been around for 5 billion years, and will probably last for another 5 before it bloats up as a red giant, sheds its outer layers and compresses down into a white dwarf, cooling down until it’s the background temperature of the Universe.

So if a galaxy like the Milky Way is just a collection of stars, isn’t that it? Doesn’t a galaxy die when its last star dies?

But you already know a galaxy is more than just stars. There’s also vast clouds of gas and dust. Some of it is primordial hydrogen left from the formation of the Universe 13.8 billion years ago.

All stars in the Milky Way formed from this primordial hydrogen. It and other similar sized galaxies produce 7 bouncing baby stars every year. Sadly, ours has used up 90% of its hydrogen, and star formation will slow down until it both figuratively, and literally, runs out of gas.

The Milky Way will die after it’s used all its star-forming gas, when all of the stars we have, and all those stars yet to be born have died. Stars like our Sun can only last for 10 billion years or so, but the smallest, coolest red dwarfs can last for a few trillion years.

The Andromeda Galaxy will collide with the Milky Way in the future. Credit: Adam Evans — The Andromeda Galaxy. Credit: Adam Evans

That should be the end, all the gas burned up and every star burned out. And that’s how it would be if our Milky Way existed all alone in the cosmos.

Fortunately, we’re surrounded by dozens of dwarf galaxies, which get merged into our Milky Way. Each merger brings in a fresh crop of stars and more hydrogen to stoke the furnaces of star formation.

There are bigger galaxies out there too. Andromeda is bearing down on the Milky Way right now, and will collide with us in the next few billion years.

When that happens, the two will merge. Then there’ll be a whole new era of star formation as the unspent gas in both galaxies mix together and are used up.

Eventually, all galaxies gravitationally bound to each other in this vicinity will merge together into a giant elliptical galaxy.

We see examples of these fossil galaxies when we look out into the Universe. Here’s M49, a supermassive elliptical galaxy. Who knows how many grand spiral galaxies stoked the fires of that gigantic cosmic engine?

Eta Carinae shines brightly in X-rays in this image from the Chandra X-Ray Observatory.

Elliptical galaxies are dead galaxies walking. They’ve used up all their reserves of star forming gas, and all that’s left are the longer lasting stars. Eventually, over vast lengths of time, those stars will wink out one after the other, until the whole thing is the background temperature of the Universe.

As long as galaxies have gas for star formation, they’ll keep thriving. Once it’s gonzo, or a dramatic merger uses all the gas in one big party, they’re on their way out.

What could we do to prolong the life of our galaxy? Let’s hear some wild speculation in the comments below.

May 28, 2015December 23, 2015 by Jason Major

Hubble Captures a Collision in a Black Hole’s “Death Star” Beam

Activity within the jet from NGC 3852 imaged by Hubble. Credit: NASA, ESA, and E. Meyer (STScI).

Even the Empire’s planet-blasting battle station has nothing compared to the immense energy being fired from the heart of NGC 3862, a supermassive black hole-harboring elliptical galaxy located 300 million light-years away.

And while jets of high-energy plasma coming from active galactic nuclei have been imaged before, for the first time activity within a jet has been observed in optical wavelengths, revealing a quite “forceful” collision of ejected material at near light speeds.

Using archived image data acquired by Hubble in 1994, 1996, and 2002 combined with new high-resolution images acquired in 2014, Eileen Meyer at the Space Telescope Science Institute (STScI) in Baltimore, Maryland identified movement in visible clumps of plasma within the jet emitted from the nucleus of NGC 3862 (aka 3C 264). One of the outwardly-moving larger clumps could be seen gaining on a slower, smaller one in front of it and the two eventually collide, creating a shockwave that brightens the resulting merged mass dramatically.

Such a collision has never been witnessed before, and certainly not thousands of light-years out from the central supermassive black hole.

Close-up image of the jet as seen in 2014. Credit: NASA, ESA, and E. Meyer (STScI).

“Something like this has never been seen before in an extragalactic jet,” Meyer said. “This will allow us a very rare opportunity to see how the kinetic energy of the collision is dissipated into radiation.”

Jets like this are created when infalling material around an active (that is, “feeding”) supermassive black hole gets caught up in its powerful spinning and twisting magnetic fields. This accelerates the material even further and, rather than permitting it to descend down past the black hole’s event horizon, results in it getting shot out into space at velocities close to the speed of light.

When material approaches the black hole in even amounts the jets are fairly consistent. But if the inflow is uneven, the jets can consist of clumps or knots traveling outward at different speeds.

Because of the motion of the galaxy itself related to our own, the speed of the clumps can appear to actually move faster than the speed of light, especially when – as seen in NGC 3862 – a large clump has already paved the way within the jet. In reality the light speed limit has not been broken, but the apparent superluminal motion so far from the SMBH indicates that the material was ejected extremely energetically.

It’s expected that the combined clusters of material will continue to brighten over the next several decades.

You can see a video of the observations below, and watch a Google+ Hangout with Hubble team members about these observations here.

Source: Hubble news center

January 26, 2015December 28, 2015 by Fraser Cain

Astronomy Cast Ep. 365: Gaia

The European Gaia spacecraft launched about a year ago with the ambitious goal of mapping one billion years in the Milky Way. That’s 1% of all the stars in our entire galaxy, which it will monitor about 70 times over its 5-year mission. If all goes well, we’ll learn an enormous amount about the structure, movements and evolution of the stars in our galaxy. It’ll even find half a million quasars.
Continue reading “Astronomy Cast Ep. 365: Gaia”

January 22, 2015December 23, 2015 by Elizabeth Howell

Why Is Our Galaxy Called The Milky Way?

This annotated artist's conception illustrates our current understanding of the structure of the Milky Way galaxy. Image Credit: NASA

We have a lot of crazy informal names for space sights. Sometimes they’re named after how they are shaped, like the Horsehead Nebula. Sometimes they have a name “borrowed” from their constellation, such as the Andromeda Galaxy. But what about our own galaxy, the Milky Way? Why does this band of stars across Earth’s sky have a name associated with food?

First, let’s back up a bit and talk a bit about what the Milky Way actually is. Astronomers believe it is a barred spiral galaxy — a galaxy with a spiral shape that has a line of stars across its middle, as you can see in the picture above. If you were to fly across the galaxy at the speed of light, it would take you an astounding 100,000 years.

The Milky Way is part of a collection of galaxies called the Local Group. We’re on a collision course with the most massive and largest member of that collection, which is the Andromeda Galaxy (also known as M31). The Milky Way is the second-largest galaxy, and the Triangulum Galaxy (M33) the third-largest. There are roughly 30 members of this group all told.

To get a sense of its immense size, you’ll be glad to hear the Earth is nowhere near the Milky Way’s center and its powerful, supermassive black hole. NASA says we’re roughly 165 quadrillion miles from the black hole, which is found in the direction of the constellation Sagittarius.

The magnetic field of our Milky Way Galaxy as seen by ESA’s Planck satellite. Credit: ESA and the Planck Collaboration.

As for how our galaxy got its name, it is indeed because of its milky appearance as it stretches across the sky. While spotting the galaxy’s arms is a challenge from our current light-polluted centers, if you get out to a more rural area it really begins to dominate the skies. The ancient Romans called our galaxy the Via Lactea, which literally means “The Road of Milk.”

And according to the Astronomy Picture of the Day website, the Greek word for “galaxy” also derives from the word “milk”. It’s hard to say if it was a coincidence, because the origin of both the Milky Way’s name and the Greek word for galaxy are long lost to prehistory, although some sources say that it was inspired by the Milky Way’s appearance.

It took thousands of years for us to understand the nature of what we were looking at. Back in the time of Aristotle, according to the Library of Congress, the Milky Way was believed to be the spot “where the celestial spheres came into contact with the terrestrial spheres.” Without a telescope, it was hard to say much more, but that began to change in the early 1600s.

Beautiful view of our Milky Way Galaxy. If other alien civilizations are out there, can we find them? Credit: ESO/S. Guisard

One important early observation, the library adds, was from the noted astronomer Galileo Galilei. (He’s best known for being credited for the discovery of four of Jupiter’s moons — Io, Europa, Callisto and Ganymede — which he spotted through a telescope.) In his 1610 volume Sidereus Nuncius, Galileo said his observations showed the Milky Way was not a uniform band, but had certain pockets with more star densities.

But the true nature of the galaxy eluded us for some time yet. Other early observations: the stars were a part of our Solar System (Thomas Wainwright, 1750 — a claim that was later shown as erroneous) and that the stars appeared to be denser on one side of the band than the other (William and John Herschel, in the late 1700s).

It took until the 20th century for astronomers to figure out that the Milky Way is just one of a large number of galaxies in the sky. This came, the library says, through a few steps: doing observations of distant “spiral nebulas” that showed their speeds were receding faster than the escape velocity of our own galaxy (Vesto Slipher, 1912); observations that a “nova” (temporary bright star) in Andromeda was fainter than our own galaxy (Herber Curtis, 1917); and most famously, Edwin Hubble’s observations of galaxies showing that they were very far from Earth indeed (1920ish).

The Hubble Ultra Deep Field seen in ultraviolet, visible, and infrared light. Image Credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI)

There are in fact more galaxies out there than we could have imagined even a century ago. Using the Hubble Space Telescope, periodically astronomers have used the powerful observatory to gaze at a tiny patch of the sky.

This has produced several “deep fields” of galaxies billions of light-years away. It’s hard to estimate just how many there are “out there”, but estimates seem to say there are at least 100 billion galaxies. That’ll keep astronomers busy observing for a while.

We have written many articles about the Milky Way for Universe Today. Here are some facts about the Milky Way, and here’s an article about the stars in the Milky Way. We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.

January 20, 2015December 23, 2015 by Elizabeth Howell

How Big Is The Milky Way?

The summertime Milky Way from Scorpius to Cygnus is broader and brighter than the winter version because we look into the direction of its center. Credit: Stephen Bockhold

The Milky Way is our home galaxy, the spot where the Earth resides. We are not anywhere near the center — NASA says we’re roughly 165 quadrillion miles from the galaxy’s black hole, for example — which demonstrates just how darn big the galaxy is. So how big is it, and how does it measure up with other neighborhood residents?

The numbers are pretty astounding. NASA estimates the galaxy at 100,000 light-years across. Since one light year is about 9.5 x 10¹²km, so the diameter of the Milky Way galaxy is about 9.5 x 10¹⁷ km in diameter. The thickness of the galaxy ranges depending on how close you are to the center, but it’s tens of thousands of light-years across.

Our galaxy is part of a collection known as the Local Group. Because some of these galaxies are prominent in our sky, the names tend to be familiar. The Milky Way is on a collision course with the most massive member of the group, called M31 or the Andromeda Galaxy. The Milky Way is the second-largest member, with M33 (the Triangulum Galaxy) the third-largest, NASA says. Andromeda appears much brighter in the night sky due to its size and relatively closer distance. There are about 30 members of this group.

Because we are inside the Milky Way’s arms, it appears as a band of stars (or a fuzzy white band) across the Earth’s sky. Casting a pair of binoculars or a telescope across it shows a mix of lighter areas and darker areas; the darker areas are dust that obscures any light from stars, galaxies and other bright objects behind it. From the outside, however, astronomers say the Milky Way is a barred spiral galaxy — a galaxy that has a band of stars across its center as well as the spiral shape.

If you’re looking for the center of the galaxy, gaze at the constellation Sagittarius, which is low on the summer sky horizon for most northern hemisphere residents. The constellation contains a massive radio source known as Sagittarius A*. Astronomers using the Chandra space telescope discovered why this supermassive black hole is relatively weak in X-rays: it’s because hot gas is being pulled inside the nebula, and most of it (99%) gets ejected and diffused.

Sagittarius A in infrared (red and yellow, from the Hubble Space Telescope) and X-ray (blue, from the Chandra space telescope). Credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

Based on observing globular clusters (star clusters) in the galaxy, astronomers have estimated the Milky Way’s overall age at 13.5 billion years old — just 200 million years younger than the rest of the universe.

However, scientists are beginning to think that different parts of the galaxy formed at different times. In 2012, for example, astronomers led by Jason Kalirai of the Space Telescope Science Institute pinned down the age of the Milky Way’s inner halo of stars: 11.5 billion years old. They used white dwarfs, the burned-out remnants of Sun-like stars, to make that measurement.

Kalirai’s group’s research indicates that the Milky Way formed in the following sequence: the halo (including globular star clusters and dwarf galaxies), the inner halo (whose stars were born as a result of this construction) and the outer halo (created when the Milky Way ate up nearby ancient dwarf galaxies).

Artist's impression of the structure of the Milky Way's halo. Credit: NASA, ESA, and A. Feild (STScI) — Artist’s impression of the structure of the Milky Way’s halo. Credit: NASA, ESA, and A. Feild (STScI)

While we’ve been focusing on the parts of the galaxy that you can see, in reality most of its mass is made up of dark matter. NASA estimates that there is about 10 times the mass of dark matter than the visible matter in the universe. (Dark matter is a form of matter that we cannot sense with conventional telescopic instruments, except through its gravitational effect on other things such as galaxies. When masses gather in high enough concentrations, they can bend the light of other objects.)

We have written many articles about the Milky Way for Universe Today. Here’s an article about the rotation of Milky Way, and here are some facts about the Milky Way. We’ve also recorded an episode of Astronomy Cast about galaxies. Listen here, Episode 97: Galaxies.

January 13, 2015December 23, 2015 by Matt Williams

One of the Milky Way’s Arms Might Encircle the Entire Galaxy

Artist's conception of the Milky Way galaxy as seen from far Galactic North (in Coma Berenices) Credit: NASA/JPL-Caltech/R. Hurt

Given that our Solar System sits inside the Milky Way Galaxy, getting a clear picture of what it looks like as a whole can be quite tricky. In fact, it was not until 1852 that astronomer Stephen Alexander first postulated that the galaxy was spiral in shape. And since that time, numerous discoveries have come along that have altered how we picture it.

For decades astronomers have thought the Milky Way consists of four arms — made up of stars and clouds of star-forming gas — that extend outwards in a spiral fashion. Then in 2008, data from the Spitzer Space Telescope seemed to indicate that our Milky Way has just two arms, but a larger central bar. But now, according to a team of astronomers from China, one of our galaxy’s arms may stretch farther than previously thought, reaching all the way around the galaxy.

This arm is known as Scutum–Centaurus, which emanates from one end of the Milky Way bar, passes between us and Galactic Center, and extends to the other side of the galaxy. For many decades, it was believed that was where this arm terminated.

However, back in 2011, astronomers Thomas Dame and Patrick Thaddeus from the Harvard–Smithsonian Center for Astrophysics spotted what appeared to be an extension of this arm on the other side of the galaxy.

*Star-forming region in interstellar space. Image credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration*

But according to astronomer Yan Sun and colleagues from the Purple Mountain Observatory in Nanjing, China, the Scutum–Centaurus Arm may extend even farther than that. Using a novel approach to study gas clouds located between 46,000 to 67,000 light-years beyond the center of our galaxy, they detected 48 new clouds of interstellar gas, as well as 24 previously-observed ones.

For the sake of their study, Sun and his colleagues relied on radio telescope data provided by the Milky Way Imaging Scroll Painting project, which scans interstellar dust clouds for radio waves emitted by carbon monoxide gas. Next to hydrogen, this gas is the most abundant element to be found in interstellar space – but is easier for radio telescopes to detect.

Combining this information with data obtained by the Canadian Galactic Plane Survey (which looks for hydrogen gas), they concluded that these 72 clouds line up along a spiral-arm segment that is 30,000 light-years in length. What’s more, they claim in their report that: “The new arm appears to be the extension of the distant arm recently discovered by Dame & Thaddeus (2011) as well as the Scutum-Centaurus Arm into the outer second quadrant.”

Ilustration of our galaxy, showing our Sun (red dot) and the possible extension of the Scutum-Centaurus Arm. CREDIT: Modified from "A Possible Extension of the Scutum-Centaurus Arm into the Outer Second Quadrant" by Yan Sun et al., in The Astrophysical Journal Letters, Vol. 798, January 2015; Robert Hurt. NASA/JPL-Caltech/SSC (background spiral). — *Illustration of our galaxy showing the possible extension of the Scutum-Centaurus Arm. CREDIT: Yan Sun/The Astrophysical Journal Letters, Vol. 798/Robert Hurt. NASA/JPL-Caltech/SSC*

This would mean the arm is not only the single largest in our galaxy, but is also the only one to effectively reach 360° around the Milky Way. Such a find would be unprecedented given the fact that nothing of the sort has been observed with other spiral galaxies in our local universe.

Thomas Dame, one of the astronomers who discovered the possible extension of the Scutum-Centaurus Arm in 2011, was quoted by Scientific American as saying: “It’s rare. I bet that you would have to look through dozens of face-on spiral galaxy images to find one where you could convince yourself you could track one arm 360 degrees around.”

Naturally, the prospect presents some problems. For one, there is an apparent gap between the segment that Dame and Thaddeus discovered in 2011 and the start of the one discovered by the Chinese team – a 40,000 light-year gap to be exact. This could mean that the clouds that Sun and his colleagues discovered may not be part of the Scutum-Centaurus Arm after all, but an entirely new spiral-arm segment.

If this is true, than it would mean that our Galaxy has several “outer” arm segments. On the other hand, additional research may close that gap (so to speak) and prove that the Milky Way is as beautiful when seen afar as any of the spirals we often observe from the comfort of our own Solar System.

Further Reading: arXiv Astrophysics, The Astrophysical Letters

December 19, 2014December 29, 2015 by Fraser Cain

Weekly Space Hangout – Dec. 19, 2014: Methane on Mars!

Host: Fraser Cain (@fcain)

Guests:
Morgan Rehnberg (cosmicchatter.org / @cosmic_chatter)
Ramin Skibba (@raminskibba)
Alessondra Springmann (@sondy)
Continue reading “Weekly Space Hangout – Dec. 19, 2014: Methane on Mars!”

December 15, 2014March 1, 2017 by Fraser Cain

Why Is Space Black?

Since there are stars and galaxies in all directions, why is space black? Shouldn’t there be a star in every direction we look?

Imagine you’re in space. Just the floating part, not the peeing into a vacuum hose or eating that funky “ice cream” from foil bags part. If you looked at the Sun, it would be bright and your retinas would crisp up. The rest of the sky would be a soothing black, decorated with tiny little less burny points of light.

If you’ve done your homework, you know that space is huge. It even be infinite, which is much bigger than huge. If it is infinite you can imagine looking out into space in any direction and there being a star. Stars would litter everything. Dumb stars everywhere wrecking the view. It’s stars all the way down, people.

So, shouldn’t the entire sky be as bright as a star, since there’s a star in every possible minute direction you could ever look in? If you’ve ever asked yourself this question, you probably won’t be surprised to know you’re not the first. Also, at this point you can tell people you were wondering about it and they’ll never know you just watched it here and then you can sound wicked smart and impress all those dudes.

This question was famously asked by the German astronomer Heinrich Wilhelm Olbers who described it in 1823. We now call this Olbers’ Paradox after him. Here let me give you a little coaching, you’ll start your conversation at the party with “So, the other day, I was contemplating Olbers’ Paradox… Oh what’s that? You don’t know what it is… oh that’s so sweet!”. The paradox goes like this: if the Universe is infinite, static and has existed forever, then everywhere you look should eventually hit a star.

Our experiences tell us this isn’t the case. So by proposing this paradox, Olbers knew the Universe couldn’t be infinite, static and timeless. It could be a couple of these, but not all three. In the 1920s, debonair man about town, Edwin Hubble discovered that the Universe isn’t static. In fact, galaxies are speeding away from us in all directions like we have the cooties.

This led to the theory of the Big Bang, that the Universe was once gathered into a single point in time and space, and then, expanded rapidly. Our Universe has proven to not be static or timeless. And so, PARADOX SOLVED!

Here’s the short version. We don’t see stars in every direction because many of the stars haven’t been around long enough for their light to get to us. Which I hope tickles your brain in the way it does mine. Not only do we have this incomprehensibly massive size of our Universe, but the scale of time we’re talking about when we do these thought experiments is absolutely boggling. So, PARADOX SOLVED!

Well, not exactly. Shortly after the Big Bang, the entire Universe was hot and dense, like the core of a star. A few hundred thousand years after the Big Bang, when the first light was able to leap out into space, everything, in every direction was as bright as the surface of a star.

Cosmic microwave background. Image credit: WMAP

So, in all directions, we should still be seeing the brightness of a star.. and yet we don’t. As the Universe expanded, the wavelengths of that initial visible light were stretched out and out and dragged to the wide end of the electromagnetic spectrum until they became microwaves. This is Cosmic Microwave Background Radiation, and you guessed it, we can detect it in every direction we can look in.

So Olbers’ instinct was right. If you look in every direction, you’re seeing a spot as bright as a star, it’s just that the expansion of the Universe stretched out the wavelengths so that the light is invisible to our eyes. But if you could see the Universe with microwave detecting eyes, you’d see this: brightness in every direction.

Did you come up with Olbers’ Paradox too? What other paradoxes have puzzled you?

December 15, 2014December 23, 2015 by Vanessa Janek

New Signal May Be Evidence of Dark Matter, Say Researchers

Dark Matter Halo and dwarf galaxies — All galaxies are thought to have a dark matter halo. This image shows the distribution of dark matter surrounding our very own Milky Way. Image credit: J. Diemand, M. Kuhlen and P. Madau (UCSC)

Dark matter is the architect of large-scale cosmic structure and the engine behind proper rotation of galaxies. It’s an indispensable part of the physics of our Universe – and yet scientists still don’t know what it’s made of. The latest data from Planck suggest that the mysterious substance comprises 26.2% of the cosmos, making it nearly five and a half times more prevalent than normal, everyday matter. Now, four European researchers have hinted that they may have a discovery on their hands: a signal in x-ray light that has no known cause, and may be evidence of a long sought-after interaction between particles – namely, the annihilation of dark matter.

When astronomers want to study an object in the night sky, such as a star or galaxy, they begin by analyzing its light across all wavelengths. This allows them to visualize narrow dark lines in the object’s spectrum, called absorption lines. Absorption lines occur because a star’s or galaxy’s component elements soak up light at certain wavelengths, preventing most photons with those energies from reaching Earth. Similarly, interacting particles can also leave emission lines in a star’s or galaxy’s spectrum, bright lines that are created when excess photons are emitted via subatomic processes such as excitement and decay. By looking closely at these emission lines, scientists can usually paint a robust picture of the physics going on elsewhere in the cosmos.

But sometimes, scientists find an emission line that is more puzzling. Earlier this year, researchers at the Laboratory of Particle Physics and Cosmology (LPPC) in Switzerland and Leiden University in the Netherlands identified an excess bump of energy in x-ray light coming from both the Andromeda galaxy and the Perseus star cluster: an emission line with an energy around 3.5keV. No known process can account for this line; however, it is consistent with models of the theoretical sterile neutrino – a particle that many scientists believe is a prime candidate for dark matter.

The researchers believe that this strange emission line could result from the annihilation, or decay, of these dark matter particles, a process that is thought to release x-ray photons. In fact, the signal appeared to be strongest in the most dense regions of Andromeda and Perseus and increasingly more diffuse away from the center, a distribution that is also characteristic of dark matter. Additionally, the signal was absent from the team’s observations of deep, empty space, implying that it is real and not just instrumental artifact.

In a pre-print of their paper, the researchers are careful to stress that the signal itself is weak by scientific standards. That is, they can only be 99.994% sure that it is a true result and not just a rogue statistical fluctuation, a level of confidence that is known as 4σ. (The gold standard for a discovery in science is 5σ: a result that can be declared “true” with 99.9999% confidence) Other scientists are not so sure that dark matter is such a good explanation after all. According to predictions made based on measurements of the Lyman-alpha forest – that is, the spectral pattern of hydrogen absorption and photon emission within very distant, very old gas clouds – any particle purporting to be dark matter should have an energy above 10keV – more than twice the energy of this most recent signal.

As always, the study of cosmology is fraught with mysteries. Whether this particular emission line turns out to be evidence of a sterile neutrino (and thus of dark matter) or not, it does appear to be a signal of some physical process that scientists do not yet understand. If future observations can increase the certainty of this discovery to the 5σ level, astrophysicists will have yet another phenomena to account for – an exciting prospect, regardless of the final result.

The team’s research has been accepted to Physical Review Letters and will be published in an upcoming issue.

November 30, 2014December 23, 2015 by Bob King

Sail Past Orion to the Outer Limits of the Milky Way

Orion (at right), Sirius (bottom) and the pale wintertime Milky Way (center) are well-placed for viewing around 11 o'clock local time in late November. Credit: Bob King

Several nights ago the chill of interstellar space refrigerated the countryside as temperatures fell well below zero. That didn’t discourage the likes of Orion and his seasonal friends Gemini, Perseus and Auriga. They only seemed to grow brighter as the air grew sharper.

Wending between these familiar constellations like a river steaming in the cold was the Milky Way. The name has always been slightly confusing as it refers to both the milky band of starlight and the galaxy itself. Every single star you see at night belongs to our galaxy, a 100,000 light-year-wide flattened disk scintillating with over 400 billion suns.

Our solar system lies in the flat plane of a barred spiral galaxy called the Milky Way. Looking through the plane, the stars pile up to form the Milky Way band. In summerr, we face toward the richer, denser core; in winter we look out toward the edge. Credit: NASA with annotations by the author — Face-on (left) and edge-on views of the Milky Way. Our solar system lies in the flat plane of a barred spiral galaxy called the Milky Way. Looking through the plane, the stars pile up to form the Milky Way band. In summer, we face toward the richer, denser core; in winter we look out toward the edge. Credit: NASA with annotations by the author

Earth, Sun and planets huddle together within the mid-plane of the disk, so that when we look straight into it, the density of stars piles up over thousands of light years to form a thick band across the sky. Since most of the stars are very distant and therefore faint, they can’t be seen individually with the naked eye. They blend together to give the Milky Way a milky or hazy look.

During a snowfall, we can see individual flakes nearby but more distant ones increase in number and blend into a uniform haze. Credit: Bob King — During a snowfall, we can see individual flakes nearby but more distant ones increase in number and blend to make a uniform haze similar to what happens when we look across the flat disk of the Milky Way. Credit: Bob King

In a snowstorm, we easily distinguish individual snowflakes falling in front of our face, but looking into the distance, the flakes blend together to create a white, foggy haze. Replace the snowflakes with stars and you have the Milky Way – with a caveat. If we lived in the center of our galaxy, the sky would be milky with stars in all directions just like that snowstorm, but since the Sun occupies the flat plane, they only appear thick when our line of sight is aimed along the galaxy’s equator. Look above and below the disk and the stars quickly thin out as our gaze pierces through the galaxy’s plane and into intergalactic space.

In this view, the ground is literally gone and we can see all around us in space. From this perspective we can see the full circle of the Milky Way. The blue line represents the galactic equator. Time is around midnight December 1st. Notice that the Sun is located in the same direction as the galaxy's center this month. Stellarium — In this view, the ground – Earth – has been removed from the picture and we can see all around us in space. Now we can see that the Milky Way band describes a full circle in the sky. The blue circle represents the galactic equator. The view shows the sky around midnight in early December. The Sun, at lower right, lies in the same direction as the galaxy’s center this month. Source: Stellarium

If you could float in space some distance from the brilliant ball of Earth, you’d see that the Milky Way band passes above, around and below you like a giant hula-hoop. Back on the ground, we can only see about two-thirds of the band over the course of a year. The other third is below the horizon and visible only from the opposite hemisphere, providing yet another good reason to make that trip to Tahiti or Ayers Rock in Australia.

Few know the winter version of the Milky Way that stands above the southeastern horizon around 10:30-11 p.m. local time on moonless nights in early December. No surprise, given it hardly compares to the brightness of the summertime version. This has much to do with where the Sun is located inside the galaxy, some 30,000 light years away from the center or more than halfway to the edge.

The opposite of the galaxy's center is the anticenter, located near El Nath in the northern horn of Taurus above the constellation Orion. Source: Stellarium — *Opposite the galaxy’s center lies the anticenter, located near El Nath in the northern horn of Taurus above the constellation Orion. Source: Stellarium*

On late fall and winter nights, our planet faces the galaxy’s outer suburbs and countryside where the stars thin out until giving way to relatively starless intergalactic space. Indeed, the anticenter of the Milky Way lies not far from the star El Nath (Beta Tauri) where Taurus meets Auriga. While the hazy band of the Milky Way is still visible through Auriga and Taurus, it’s thin and anemic compared to summer’s billowy star clouds.

At nightfall in July and August, we face toward the galaxy’s center where 30,000 light years worth of stars, star clouds and nebulae stack up to fatten the Milky Way into a bright, chunky arch on summer evenings compared to winter’s thin gruel.

The slanting winter Milky Way touches many of the familiar, bright constellations of the December sky. This map shows the sky facing southeast around 11 o'clock local time in early December or 9 p.m. in late December. Source: Stellarium — The slanting winter Milky Way touches many of the familiar, bright constellations of December. This map shows the sky facing southeast around 11 o’clock local time in early December or 9 p.m. in late December. Source: Stellarium

The winter Milky Way starts east of brilliant Sirius and grazes the east side of Orion before ascending into Gemini and Auriga and arching over into the western sky to Cassiopeia’s “W”. Binoculars and telescopes resolve it into individual stars and star clusters and help us appreciate what a truly beautiful and rich place our galactic home is.

Few sights that impress us with the scope and scale of where we live than seeing the Milky Way under a dark sky during the silence of a winter night. Picture Earth and yourself as members of that glowing carpet of stars, and when you can’t take the cold anymore, enjoy the delicious pleasure of stepping inside to unwrap and warm up. You’ve been on a long journey.