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Navigating Deep Space by Starlight
On August 6, 1967, astrophysicist Jocelyn Bell Burnell noticed a blip in her radio telescope data. And then another. Eventually, Bell Burnell figured out that these blips, or pulses, were not from people or machines.
The blips were constant. There was something in space that was pulsing in a regular pattern, and Bell Burnell figured out that it was a pulsar: a rapidly spinning neutron star emitting beams of light. Neutron stars are superdense objects created when a massive star dies. Not only are they dense, but neutron stars can also spin really fast! Every star we observe spins, and due to a property called angular momentum, as a collapsing star gets smaller and denser, it spins faster. It’s like how ice skaters spin faster as they bring their arms closer to their bodies and make the space that they take up smaller.
The pulses of light coming from these whirling stars are like the beacons spinning at the tops of lighthouses that help sailors safely approach the shore. As the pulsar spins, beams of radio waves (and other types of light) are swept out into the universe with each turn. The light appears and disappears from our view each time the star rotates.
After decades of studying pulsars, astronomers wondered—could they serve as cosmic beacons to help future space explorers navigate the universe? To see if it could work, scientists needed to do some testing!
First, it was important to gather more data. NASA’s NICER, or Neutron star Interior Composition Explorer, is a telescope that was installed aboard the International Space Station in 2017. Its goal is to find out things about neutron stars like their sizes and densities, using an array of 56 special X-ray concentrators and sensitive detectors to capture and measure pulsars’ light.
But how can we use these X-ray pulses as navigational tools? Enter SEXTANT, or Station Explorer for X-ray Timing and Navigation Technology. If NICER was your phone, SEXTANT would be like an app on it.
During the first few years of NICER’s observations, SEXTANT created an on-board navigation system using NICER’s pulsar data. It worked by measuring the consistent timing between each pulsar’s pulses to map a set of cosmic beacons.
When calculating position or location, extremely accurate timekeeping is essential. We usually rely on atomic clocks, which use the predictable fluctuations of atoms to tick away the seconds. These atomic clocks can be located on the ground or in space, like the ones on GPS satellites. However, our GPS system only works on or close to Earth, and onboard atomic clocks can be expensive and heavy. Using pulsar observations instead could give us free and reliable “clocks” for navigation. During its experiment, SEXTANT was able to successfully determine the space station’s orbital position!
We can calculate distances using the time taken for a signal to travel between two objects to determine a spacecraft’s approximate location relative to those objects. However, we would need to observe more pulsars to pinpoint a more exact location of a spacecraft. As SEXTANT gathered signals from multiple pulsars, it could more accurately derive its position in space.
So, imagine you are an astronaut on a lengthy journey to the outer solar system. You could use the technology developed by SEXTANT to help plot your course. Since pulsars are reliable and consistent in their spins, you wouldn’t need Wi-Fi or cell service to figure out where you were in relation to your destination. The pulsar-based navigation data could even help you figure out your ETA!
None of these missions or experiments would be possible without Jocelyn Bell Burnell’s keen eye for an odd spot in her radio data decades ago, which set the stage for the idea to use spinning neutron stars as a celestial GPS. Her contribution to the field of astrophysics laid the groundwork for research benefitting the people of the future, who yearn to sail amongst the stars.
Keep up with the latest NICER news by following NASA Universe on X and Facebook and check out the mission’s website. For more on space navigation, follow @NASASCaN on X or visit NASA’s Space Communications and Navigation website.
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Navigating Space by the Stars
A sextant is a tool for measuring the angular altitude of a star above the horizon and has helped guide sailors across oceans for centuries. It is now being tested aboard the International Space Station as a potential emergency navigation tool for guiding future spacecraft across the cosmos. The Sextant Navigation investigation will test the use of a hand-held sextant that utilizes star sighting in microgravity.
Read more about how we’re testing this tool in space!
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Gravitational Waves in the Space-Time Continuum
Einstein's Theories of Relativity
Einstein has two theories of relativity. The first is The Theory of Special Relativity (1905). This is a theory of mechanics that correctly describes the motions of objects moving near the speed of light. This theory predicts that mass increases with velocity. The equation is E=MC^2 or Energy = Mass × Speed of Light ^2.
In 1916, Einstein proposed the Theory of General Relativity, which generalized his Theory of Special Relativity and had the first predictions of gravitational waves. It implied a few things.
Space-Time is a 4-Dimensional continuum.
Principle of equivalence of gravitational and inertial mass.
This suggests that Mass-Energy distorts the fabric of space-time in a predictable way (gravitational waves). It also implies
Strong gravitational force makes time slow down.
Light is altered by gravity
Gravity in strong gravitational fields will no longer obey Newton's Inverse-Square Law.
What is Newton's Inverse-Square Law?
Newton's Inverse-Square Law suggests that the force of gravity between any two objects is inversely proportional to the square of the separation distance between the two centers.
Stephen Hawking's Theory of Everything
Stephen Hawking's Theory of Everything is the solution to Einstein's equation in his Theory of General Relativity. It says that the mass density of the universe exceeds the critical density.
Critical Density: amount of mass needed to make a universe adopt a flat geometry.
This theory states that when the universe gets too big it will crash back into its center in a "Big Crunch" creating giant black hole. The energy from this "Big Crunch" will rebound and create a new "Big Bang".
Big Crunch: hypothetical scenario for the end of the known universe. The expansion of the universe will reverse and collapse on itself. The energy generated will create a new Big Bang, creating a new universe.
Big Bang: Matter will expand from a single point from a state of high density and matter. This will mark the birth of a new universe.
Basic Facts about Gravitational Waves
Invisible "ripples" in the Space-Time Continuum
Travel at the speed of light
186,000 miles per second / 299,337.984 Kilometers per second
11,160,000 miles per minute / 17,960,279.04 Kilometers per minute
669,600,000 miles per hour / 1,077,616,742.4 Kilometers per hour
There are four (4) defined categories
Continuous
Stochastic
Burst
Compact Binary Inspiral
What is LIGO?
The first proof of the existence of gravitational waves came in 1974. 20+ years after Einstein's death.
The first physical proof came in 2015, 100 years after his theory was published. The waves were detected by LIGO.
LIGO- Laser Interferometer Gravitational-Wave Observatory
The waves detected in 2015 came from 2 black holes that collided 1.3 billion years ago in the constellation Hydra. 1.3 billion years ago multicellular life was just beginning to spread on Earth, it was before the time of the dinosaurs!
Continuous Gravitational Waves
Produced by a single spinning massive object.
Caused by imperfections on the surface.
The spin rate of the object is constant. The waves are come at a continuous frequency.
Stochastic Gravitational Waves
Smalles waves
Hardest to detect
Possibly caused by remnants of gravitational radiation left over from the Big Bang
Could possibly allow us to look at the history of the Universe.
Small waves from every direction mixed together.
Burst Gravitational Waves
Never been detected.
Like ever.
Never ever.
Not once.
Nope
No
N E V E R
We don't know anything about them.
If we learn about them they could reveal the greatest revolutionary information about the universe.
Compact Binary Inspiral Gravitational Waves
All waves detected by LIGO fall into this category.
Produced by orbiting pairs of massive and dense objects. (Neutron Stars, Black Holes)
Three (3) subclasses
Binary Neutron Star (BNS) // Two (2) Neutron Stars colliding
Binary Black Hole (BBH) // Two (2) Black Holes colliding
Neutron Star- Black Hole Binary (NSBH) // A black hole and a neutron star colliding
Each subclass creates its own unique wave pattern.
Waves are all caused by the smae mechanism called an "inspiral".
Occur over millions of years.
Over eons the objects orbit closer together.
The closer they get, the faster they spin.
Sources Used:
On The Shoulders Of Giants by Stephen Hawking
Oxford Astronomy Encyclopedia
@watch-out-idiot-passing-through @nasa
Your fave is problematic: Neutron Stars
Neutron stars are probably one of the weirdest type of objects to exist in the universe… but first let me explain what a neutron star is
when a star with the mass of 8-20 times of the sun dies (and by dies I mean fucking explodes), the core collapses to form a neutron star
they are incredibly dense, spin rapidly and have very strong magnetic fields
sounds all fun and games, right? sounds normal? well listen up
So, we know that electrons usually refuse to be squeezed together. but in a death event of a big star, the pressure is so extreme that protons and electrons get violently SMASHED together and form neutrons.
sounds like someone needs to take an anti-agression class if you ask me
Now, what once was a star more massive than the Sun, is condensed to a tiny ball (usually about 10-20km!) of neutrons, with all of the mass in this tiny ball.
To visualize, imagine the mass of the Sun (300 000X the mass of the Earth), in a little 20km sphere, the size of a small city.
To visualize the density of a neutron star, think of the classic model of the atom. if an atom was a sports field 100m across, it would be mostly empty. almost all of the atom’s mass sits in the core, in this example, the core is the size of a marble.
but in a neutron star, this doesn’t apply anymore. in a neutron star, the entire stadium would be filled to the brim with neutrons. ALL. OF. IT.
a single cubic centimetre of Neutronium has the mass of 400 million tons. that’s the total mass of every single car and truck in the US.
the typical gravity of a neutron star is about 100 million times of that of the Earth. clingy as shit
so far, we have detected over 1000 of these weird fucks in our galaxy alone. yikes
some Neutron stars are vampires. They can be in a binary star system where a normal star orbits them and they feed of that material
summary: extremely weird and violent space ball of rage, tiny, filled to the top with anger, sometimes a vampire
What's a neutron star? I read about them in Bill Bryson's book, but I couldn't figure out why a neutron start would happen in the first place?
When massive stars collapse, the core of the star gets compressed extremely tightly by the force of its own gravity. As the core collapses, the electrons and protons in the core get closer and closer together. Eventually, the core gets so dense that the electrons and protons are forced together, combining into neutrons. The entire core becomes essentially a solid ball of neutrons, as dense as an atomic nuclei. The outer layers of the star, which are also rushing in towards the core, bounce off of this rock-hard layer of neutrons and whiz off into space, creating a supernova and leaving behind a neutron star at the center. And all of this happens in less than a second. Pretty wild. To summarize: neutron stars are giant balls of neutrons that resulted when a stellar core collapsed and became so dense the protons and electrons combined into neutrons.
Side note: Robert L. Forward wrote a really interesting novel called Dragon’s Egg, which was about intelligent life on a neutron star! It’s quite an interesting read, and you learn a whole lot about neutron stars since the author has a Ph.D in physics. If you want a copy, you can find it here; you won’t find it at a bookstore because it’s out of print, but you can find a used copy online (I linked to one). Let me know if you have any other questions, I’m happy to answer them!
Neutron Stars Collide
The recent detection of two neutron stars colliding has sent waves through spacetime and the astronomical community.
You may have seen headlines in the news and not really know why this is such a big deal.
Here’s the Sparknotes version:
A while back a thing called “gravitational waves” were observed for the first time. These are fluctuations in the fabric of spacetime that propagate out from their source just like light, i.e. radially/like a pebble dropped in water. General relativity shows us that the acceleration of objects with mass cause this event to occur.
Until fairly recently these have been too difficult to observe and in fact Einstein didn’t think we’d ever be able to. A series of laser interferometers have disproved that assumption. Using high-precision analysis of how the lasers shift as a gravitational wave moves through them scientists can now see the small movements in the universe that are gravitational waves.
Importantly, we now have three such observatories capable of working together. Known as the “LIGO-Virgo” team, two observatories in the U.S. and one in Italy detected these shifting in spacetime. Three is a pretty magical number in coordinating detections like this because you can then triangulate where the signal comes from and…
BINGO! Within hours optical observatories were zeroing in on the predicted source of these spacetime fluctuations. Indeed, they confirmed the presence of a previously unseen glow:
What you’re looking at is the glow of two neutron stars colliding into each other. This explosion has the energy of approximately 260,000,000 suns.
Each of these stars has such a large mass that the waves in spacetime are actually detectable from a distance of 140 million light-years away.
Impressive, right? Although you might agree, this still begs the question of what exactly we’ve learned from this event. Well - a lot. Since this is the first observation we have made with both gravitational wave observatories and more traditional astronomical observatories (i.e. light detecting ones) we’ve been able to put some numbers on the phenomenon. Here are some of the things we’ve learned:
1) Gravitational waves propagate at the speed of light!
2) A huge portion of heavier elements (like gold and uranium!) may have their origins in neutron star collisions! Nuclear synthesis in stars more typical like the sun is restricted to closer to 10% of the star’s mass being able to fuse elements together into new ones. This process is actually quite inefficient (actually, YOU are a more efficient radiator than the sun!) and it becomes more difficult for a star to fuse the heavier elements. Before this event we didn’t have a good way of explaining why we found so much more heavy stuff than stellar nuclear synthesis could account for. Now? Baboom! This neutron star collision resulted in the synthesis of so much gold that it’s about 150 times more massive than the Earth! Cha-Ching!
So if you’re an amateur (or professional, I suppose) astronomer and you want to see this collision, now dubbed GW170817, and you build a little radio telescope (another post!), you’ll be able to detect this collision for the next 5-10 years due to the afterglow.
(Image credit: NASA and ESA)
Gamma-ray Bursts: Black Hole Birth Announcements
Gamma-ray bursts are the brightest, most violent explosions in the universe, but they can be surprisingly tricky to detect. Our eyes can’t see them because they are tuned to just a limited portion of the types of light that exist, but thanks to technology, we can even see the highest-energy form of light in the cosmos — gamma rays.
So how did we discover gamma-ray bursts?
Accidentally!
We didn’t actually develop gamma-ray detectors to peer at the universe — we were keeping an eye on our neighbors! During the Cold War, the United States and the former Soviet Union both signed the Nuclear Test Ban Treaty of 1963 that stated neither nation would test nuclear weapons in space. Just one week later, the US launched the first Vela satellite to ensure the treaty wasn’t being violated. What they saw instead were gamma-ray events happening out in the cosmos!
Things Going Bump in the Cosmos
Each of these gamma-ray events, dubbed “gamma-ray bursts” or GRBs, lasted such a short time that information was very difficult to gather. For decades their origins, locations and causes remained a cosmic mystery, but in recent years we’ve been able to figure out a lot about GRBs. They come in two flavors: short-duration (less than two seconds) and long-duration (two seconds or more). Short and long bursts seem to be caused by different cosmic events, but the end result is thought to be the birth of a black hole.
Short GRBs are created by binary neutron star mergers. Neutron stars are the superdense leftover cores of really massive stars that have gone supernova. When two of them crash together (long after they’ve gone supernova) the collision releases a spectacular amount of energy before producing a black hole. Astronomers suspect something similar may occur in a merger between a neutron star and an already-existing black hole.
Long GRBs account for most of the bursts we see and can be created when an extremely massive star goes supernova and launches jets of material at nearly the speed of light (though not every supernova will produce a GRB). They can last just a few seconds or several minutes, though some extremely long GRBs have been known to last for hours!
A Gamma-Ray Burst a Day Sends Waves of Light Our Way!
Our Fermi Gamma-ray Space Telescope detects a GRB nearly every day, but there are actually many more happening — we just can’t see them! In a GRB, the gamma rays are shot out in a narrow beam. We have to be lined up just right in order to detect them, because not all bursts are beamed toward us — when we see one it’s because we’re looking right down the barrel of the gamma-ray gun. Scientists estimate that there are at least 50 times more GRBs happening each day than we detect!
So what’s left after a GRB — just a solitary black hole? Since GRBs usually last only a matter of seconds, it’s very difficult to study them in-depth. Fortunately, each one leaves an afterglow that can last for hours or even years in extreme cases. Afterglows are created when the GRB jets run into material surrounding the star. Because that material slows the jets down, we see lower-energy light, like X-rays and radio waves, that can take a while to fade. Afterglows are so important in helping us understand more about GRBs that our Neil Gehrels Swift Observatory was specifically designed to study them!
Last fall, we had the opportunity to learn even more from a gamma-ray burst than usual! From 130 million light-years away, Fermi witnessed a pair of neutron stars collide, creating a spectacular short GRB. What made this burst extra special was the fact that ground-based gravitational wave detectors LIGO and Virgo caught the same event, linking light and gravitational waves to the same source for the first time ever!
For over 10 years now, Fermi has been exploring the gamma-ray universe. Thanks to Fermi, scientists are learning more about the fundamental physics of the cosmos, from dark matter to the nature of space-time and beyond. Discover more about how we’ll be celebrating Fermi’s achievements all year!
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Black Holes are NICER Than You Think!
We’re learning more every day about black holes thanks to one of the instruments aboard the International Space Station! Our Neutron star Interior Composition Explorer (NICER) instrument is keeping an eye on some of the most mysterious cosmic phenomena.
We’re going to talk about some of the amazing new things NICER is showing us about black holes. But first, let’s talk about black holes — how do they work, and where do they come from? There are two important types of black holes we’ll talk about here: stellar and supermassive. Stellar mass black holes are three to dozens of times as massive as our Sun while supermassive black holes can be billions of times as massive!
Stellar black holes begin with a bang — literally! They are one of the possible objects left over after a large star dies in a supernova explosion. Scientists think there are as many as a billion stellar mass black holes in our Milky Way galaxy alone!
Supermassive black holes have remained rather mysterious in comparison. Data suggest that supermassive black holes could be created when multiple black holes merge and make a bigger one. Or that these black holes formed during the early stages of galaxy formation, born when massive clouds of gas collapsed billions of years ago. There is very strong evidence that a supermassive black hole lies at the center of all large galaxies, as in our Milky Way.
Imagine an object 10 times more massive than the Sun squeezed into a sphere approximately the diameter of New York City — or cramming a billion trillion people into a car! These two examples give a sense of how incredibly compact and dense black holes can be.
Because so much stuff is squished into such a relatively small volume, a black hole’s gravity is strong enough that nothing — not even light — can escape from it. But if light can’t escape a dark fate when it encounters a black hole, how can we “see” black holes?
Scientists can’t observe black holes directly, because light can’t escape to bring us information about what’s going on inside them. Instead, they detect the presence of black holes indirectly — by looking for their effects on the cosmic objects around them. We see stars orbiting something massive but invisible to our telescopes, or even disappearing entirely!
When a star approaches a black hole’s event horizon — the point of no return — it’s torn apart. A technical term for this is “spaghettification” — we’re not kidding! Cosmic objects that go through the process of spaghettification become vertically stretched and horizontally compressed into thin, long shapes like noodles.
Scientists can also look for accretion disks when searching for black holes. These disks are relatively flat sheets of gas and dust that surround a cosmic object such as a star or black hole. The material in the disk swirls around and around, until it falls into the black hole. And because of the friction created by the constant movement, the material becomes super hot and emits light, including X-rays.
At last — light! Different wavelengths of light coming from accretion disks are something we can see with our instruments. This reveals important information about black holes, even though we can’t see them directly.
So what has NICER helped us learn about black holes? One of the objects this instrument has studied during its time aboard the International Space Station is the ever-so-forgettably-named black hole GRS 1915+105, which lies nearly 36,000 light-years — or 200 million billion miles — away, in the direction of the constellation Aquila.
Scientists have found disk winds — fast streams of gas created by heat or pressure — near this black hole. Disk winds are pretty peculiar, and we still have a lot of questions about them. Where do they come from? And do they change the shape of the accretion disk?
It’s been difficult to answer these questions, but NICER is more sensitive than previous missions designed to return similar science data. Plus NICER often looks at GRS 1915+105 so it can see changes over time.
NICER’s observations of GRS 1915+105 have provided astronomers a prime example of disk wind patterns, allowing scientists to construct models that can help us better understand how accretion disks and their outflows around black holes work.
NICER has also collected data on a stellar mass black hole with another long name — MAXI J1535-571 (we can call it J1535 for short) — adding to information provided by NuSTAR, Chandra, and MAXI. Even though these are all X-ray detectors, their observations tell us something slightly different about J1535, complementing each other’s data!
This rapidly spinning black hole is part of a binary system, slurping material off its partner, a star. A thin halo of hot gas above the disk illuminates the accretion disk and causes it to glow in X-ray light, which reveals still more information about the shape, temperature, and even the chemical content of the disk. And it turns out that J1535’s disk may be warped!
Image courtesy of NRAO/AUI and Artist: John Kagaya (Hoshi No Techou)
This isn’t the first time we have seen evidence for a warped disk, but J1535’s disk can help us learn more about stellar black holes in binary systems, such as how they feed off their companions and how the accretion disks around black holes are structured.
NICER primarily studies neutron stars — it’s in the name! These are lighter-weight relatives of black holes that can be formed when stars explode. But NICER is also changing what we know about many types of X-ray sources. Thanks to NICER’s efforts, we are one step closer to a complete picture of black holes. And hey, that’s pretty nice!
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