APOD: Zeta Oph: Runaway Star (2024 Jan 04)

[APOD Robot]

Zeta Oph: Runaway Star

Explanation: Like a ship plowing through cosmic seas, runaway star Zeta Ophiuchi produces the arcing interstellar bow wave or bow shock seen in this stunning infrared portrait. In the false-color view, bluish Zeta Oph, a star about 20 times more massive than the Sun, lies near the center of the frame, moving toward the left at 24 kilometers per second. Its strong stellar wind precedes it, compressing and heating the dusty interstellar material and shaping the curved shock front. What set this star in motion? Zeta Oph was likely once a member of a binary star system, its companion star was more massive and hence shorter lived. When the companion exploded as a supernova catastrophically losing mass, Zeta Oph was flung out of the system. About 460 light-years away, Zeta Oph is 65,000 times more luminous than the Sun and would be one of the brighter stars in the sky if it weren't surrounded by obscuring dust. The image spans about 1.5 degrees or 12 light-years at the estimated distance of Zeta Ophiuchi. In January 2020, NASA placed the Spitzer Space Telescope in safe mode, ending its 16 successful years of exploring the cosmos.

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[Ann]

Zeta Oph: Runaway Star. Image Credit: NASA, JPL-Caltech, Spitzer Space Telescope

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Today's APOD is an old image, from 2012. But it's not as old as the picture of galaxy NGC 1232, a picture from 1998 that was the APOD for January 1, 2024.

Anyway. Disregarding the infrared bow shock for a moment (or I may not talk about it at all ), Zeta Ophiuchi, the star of the show, has also created a large, if somewhat faint, red hydrogen alpha emission nebula surrounding itself:

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The Zeta Ophiuchi nebula is the large red blob at upper left. Below it, you can see the red-yellow-blue nebula surrounding stars Tau Scorpii (red nebula), Antares (yellow nebula) and Rho Ophiuchi (blue nebula). Above blue Rho Ophiuchi you can see another blue nebula, the Blue Horse, and to the right of the Blue Horse is some red nebulosity surrounding stars Delta and Pi Scorpii.

The way I understand it, Zeta Ophiuchi is moving "up and to the left", so it's tempting to think that its birthplace was close to the Antares/Rho Ophiuchi complex.

Let's get back to the concept of runaway stars, shall we? There are some famous and quite blue and massive runaway stars in the sky. The most famous runaways are probably AE Aurigae, 53 Arietis and Mu Columbae, because they are three massive blue stars that were all ejected from the Iota Orionis region just below the Orion Nebula.

Based on their common high speed motion, astronomers have drawn the paths of AE Aurigae, Mu Columbae and 53 Arietis, all 6th-magnitude stars easily visible in binoculars, back to the Orion Nebula region. Credit: Sky & Telescope/Stellarium

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Iota Orionis, which may have interacted with a binary Mu Col and AE Aur in the distant past, can be found today immediately south of the Orion Nebula. Credit: Sky & Telescope/Stellarium

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Sky & Telescope wrote:

Not only do Mu Col and AE Aur share the same 230,000 mph (100 km/sec) space velocity, they're racing away from one another in almost exactly opposite directions. Just as we trace meteor trails backwards to their radiant, we can trace the paths of Mu Col and AE Aur 2.5 million years back in time. They "meet" in the rich, star-forming regions highlighted by today's Orion Nebula. Today, they're 1,600 light-years apart. Back then, the nebula had only begun to emerge from darkness, awoken by the flickering light of its earliest suns.
...
So what caused the three to abruptly pick up and leave the scene? A supernova explosion might have sent the stars reeling in opposite directions, but recently discovered evidence points to gravitational interactions within a compact star cluster. It's thought that Mu Col and AE Aur once shared company as a binary system that collided or strayed too close to another binary pair. Complicated gravitational interactions ensued, breaking the binary's bond and flinging the stars into space as "runaways" while leaving the other binary intact, still lurking near the scene of the crime.

AE Aurigae has recently blundered into a gas cloud during its headlong flight through space, and, while it is visiting this cloud, it is lighting it up and ionizing it.

AE Aurigae and the Flaming Star Nebula. Credit & Copyright: Amir Abolfath

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AE Aurigae and the Flaming Star Nebula. Credit: Jorge Garcia.

AE Aurigae is moving almost due north, but in the picture above left, north is to the lower right. You can see how the star creates "chaos" in the nebula as it plunges into it, while long streamers of gas seem to fly away from it at left. In the picture at right, north is at left, and the star is moving left.

Ah, but AE Aurigae is making its own bow shock too, as you can see in this picture from NASA's WISE spacecraft.


And speaking about bow shocks, isn't this a perfectly elegant specimen?

Blue supergiant HD 77581 and its compact companion, Vela X-1. Vela X-1 may be a neutron star created in a supernova explosion, and the Vela X-1 progenitor may have exploded 'asymmetrically' and given itself a kick that turned it (and its high-mass companion) into a runaway pair. Credit: ESO.

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A runaway star has also been discovered apparently kicked out of the humongously large and massive R 136a cluster in the Large Magellanic Cloud:

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One of my favorite runaway stars is Zeta Puppis, a rare star because it is so hot:

Illustration of Zeta Puppis. Credit: Kryptid.

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Wikipedia wrote:

Zeta Puppis has been extensively studied because of the rarity of such hot massive stars and its relative closeness to Earth, but its physical parameters and distance are still poorly known. It would be a valuable step on the cosmic distance ladder, clarifying the distance of other high luminosity stars in the Milky Way galaxy and external galaxies.

The spectral type is O4If(n)p. O4 indicates a hot massive hydrogen-burning star, typically 40,000–44,000K.[6][5][8] The "f" indicates that the spectrum has emission lines of ionised Helium and Nitrogen, not uncommon in somewhat evolved hot O stars and typically identified by the composite emission and absorption profile of the 468.6nm HeII spectral line. The "n" (for nebulous) indicates broadened absorption lines, caused by rapid rotation of the star, in this case over 220 km/s at the equator. The "p" is a general spectral indicator of peculiarity. This combination of spectral characters is unusual because evolved hot stars are expected to rotate relatively slowly after braking by a strong stellar wind, and only 8 stars of this type are known in the Milky Way.

Yeah. Runaway stars are fun! There is a list of some of them here, although it is not really up to date.

Ann

[Ann]

Wait! I forgot one of the most remarkable runaway stars in the sky, the very cool, teetering-on-the-brink-of-stellar-death-and-white-dwarfhood, the "wonderful" Mira!

Ultraviolet mosaic of Mira's bow shock and tail obtained using NASA's Galaxy Evolution Explorer (GALEX)

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Wikipedia wrote:

Ultraviolet studies of Mira by NASA's Galaxy Evolution Explorer (GALEX) space telescope have revealed that it sheds a trail of material from the outer envelope, leaving a tail 13 light-years in length, formed over tens of thousands of years.[27][28] It is thought that a hot bow wave of compressed plasma/gas is the cause of the tail; the bow wave is a result of the interaction of the stellar wind from Mira A with gas in interstellar space, through which Mira is moving at an extremely high speed of 130 kilometres per second (290,000 miles per hour).[29] The tail consists of material stripped from the head of the bow wave, which is also visible in ultraviolet observations. Mira's bow shock will eventually evolve into a planetary nebula, the form of which will be considerably affected by the motion through the interstellar medium (ISM).[30] Mira’s tail offers a unique opportunity to study how stars like our sun die and ultimately seed new solar systems. As Mira hurls along, its tail drops off carbon, oxygen and other important elements needed for new stars, planets, and possibly even life to form. This tail material, visible now for the first time, has been shed over the past 30,000 years.

So Mira is a cool cool star with a long long tail of hot hot plasma! That's sure something.

Ann

[bls0326]

Ann - Many thanks for all the colorful runaway star pictures and the descriptions!

Brian

[johnnydeep]

How does a runaway star start moving? The text posits that a supernova of Rho Oph's likely binary companion "flung" it out of the system, but how? Was it simply due to high speed gas from the explosion colliding with it? Or was it more of a gravitational effect: loss of mass of the companion no longer able to hold Rho Oph in orbit? (though something tells me that's not really how the gravitational dynamics would work )

[Roy]

How does a runaway star start moving? The text posits that a supernova of Rho Oph's likely binary companion "flung" it out of the system, but how? Was it simply due to high speed gas from the explosion colliding with it? Or was it more of a gravitational effect: loss of mass of the companion no longer able to hold Rho Oph in orbit? (though something tells me that's not really how the gravitational dynamics would work )

Yes, there is a lot of use of that verb “to fling”. It translates to “something started to move a star (massive) really fast (lots of kinetic energy) but we don’t know what”. As a kid, I fooled around with a sling. They are hard to aim, but one can get surprising distance. So we are looking for evidence of a cosmic slinger.

[Chris Peterson]

How does a runaway star start moving? The text posits that a supernova of Rho Oph's likely binary companion "flung" it out of the system, but how? Was it simply due to high speed gas from the explosion colliding with it? Or was it more of a gravitational effect: loss of mass of the companion no longer able to hold Rho Oph in orbit? (though something tells me that's not really how the gravitational dynamics would work )

Likely mechanisms are readily modeled. In a close binary system, gravitational wave radiation results in the bodies getting closer together, and eventually their Roche lobes overlap and mass is transferred from the donor to the accretor. This results in a transfer of angular momentum, which can either draw the two closer, or push them apart. When the accretor goes SN, the mass distribution of the system changes radically, so angular momentum can be conserved by changing the component velocities. If you're interested in details (the math is actually not too bad) you can find a variety of recent papers on the topic. Take a look at https://www.aanda.org/articles/aa/full_ ... 92-20.html for a nice discussion.

(The key point is that the ejection velocity is created by orbital dynamic mechanisms, not by some kind of "pushing" from the SN front.)

[Ann]

Take a look at this star cluster simulation:

Click to play embedded YouTube video.

Note that at 0.32 or 0.33, two stars are ejected from the cluster and leave it in opposite directions. The most likely explanation is that these two stars formed a binary that interacted with another binary in such a way that the two stars in the gravitational bond between the stars of the first binary was broken up, and the stars moved away from each other in the directions of the tangent, in opposite directions.

This is a likely way to create runaway stars, I'd say.

Ann

[johnnydeep]

How does a runaway star start moving? The text posits that a supernova of Rho Oph's likely binary companion "flung" it out of the system, but how? Was it simply due to high speed gas from the explosion colliding with it? Or was it more of a gravitational effect: loss of mass of the companion no longer able to hold Rho Oph in orbit? (though something tells me that's not really how the gravitational dynamics would work )

Likely mechanisms are readily modeled. In a close binary system, gravitational wave radiation results in the bodies getting closer together, and eventually their Roche lobes overlap and mass is transferred from the donor to the accretor. This results in a transfer of angular momentum, which can either draw the two closer, or push them apart. When the accretor goes SN, the mass distribution of the system changes radically, so angular momentum can be conserved by changing the component velocities. If you're interested in details (the math is actually not too bad) you can find a variety of recent papers on the topic. Take a look at https://www.aanda.org/articles/aa/full_ ... 92-20.html for a nice discussion.

(The key point is that the ejection velocity is created by orbital dynamic mechanisms, not by some kind of "pushing" from the SN front.)

Thanks! I'll have to try to understand that paper. So it's not at all akin to a bomb going off near a rock and sending the rock flying due to what I presume is the shockwave generated in the atmosphere. Hmm, if the same event happened in orbit, I suppose the rock would not be affected nearly as much due to the lack of atmosphere, though there would still be some momentum transfer from the expanding explosive gas...right?

And, PS - seriously, is there nothing you don't know?!

[johnnydeep]

Take a look at this star cluster simulation:

Click to play embedded YouTube video.

Note that at 0.32 or 0.33, two stars are ejected from the cluster and leave it in opposite directions. The most likely explanation is that these two stars formed a binary that interacted with another binary in such a way that the two stars in the gravitational bond between the stars of the first binary was broken up, and the stars moved away from each other in the directions of the tangent, in opposite directions.

This is a likely way to create runaway stars, I'd say.

Ann

Thanks. So the "flinging" mechanism could be either gravitational interaction with another binary (or perhaps lone star), or as Chris explained above, a complex momentum/gravitational interaction between the runaway and its supernova companion.

[Ann]

(The key point is that the ejection velocity is created by orbital dynamic mechanisms, not by some kind of "pushing" from the SN front.)

Do I take that to mean that the star going supernova sheds a lot of mass during its explosion, so that the dramatic weight-loss supernova remnant can't hold on to its companion the way it could before it exploded (when it was a lot more massive)?

Or does it mean, perhaps, that the supernova exploded "asymmetrically", sending perhaps both the compact remnant (if there is one) and its binary companion rushing away in the opposite direction?

Ann

[Chris Peterson]

Take a look at this star cluster simulation:

Click to play embedded YouTube video.

Note that at 0.32 or 0.33, two stars are ejected from the cluster and leave it in opposite directions. The most likely explanation is that these two stars formed a binary that interacted with another binary in such a way that the two stars in the gravitational bond between the stars of the first binary was broken up, and the stars moved away from each other in the directions of the tangent, in opposite directions.

This is a likely way to create runaway stars, I'd say.

Ann

It is. Also, interactions of stars with the center of our galaxy. But that's a different mechanism entirely than runaway stars created by supernovas.

[Chris Peterson]

(The key point is that the ejection velocity is created by orbital dynamic mechanisms, not by some kind of "pushing" from the SN front.)

Do I take that to mean that the star going supernova sheds a lot of mass during its explosion, so that the dramatic weight-loss supernova remnant can't hold on to its companion the way it could before it exploded (when it was a lot more massive)?

Or does it mean, perhaps, that the supernova exploded "asymmetrically", sending perhaps both the compact remnant (if there is one) and its binary companion rushing away in the opposite direction?

Ann

My understanding is that it involves the transfer of angular momentum while the stars' Roche lobes overlap during the SN of one of them.

[Avalon]

So are there sequences of photographs that can show the runaway stars moving through space? Moving apart from each other or nearby objects? Is that how their velocities are calculated? What if they are heading more toward earth or away from it? Can their movements still be measured?

[Ann]

(The key point is that the ejection velocity is created by orbital dynamic mechanisms, not by some kind of "pushing" from the SN front.)

Do I take that to mean that the star going supernova sheds a lot of mass during its explosion, so that the dramatic weight-loss supernova remnant can't hold on to its companion the way it could before it exploded (when it was a lot more massive)?

Or does it mean, perhaps, that the supernova exploded "asymmetrically", sending perhaps both the compact remnant (if there is one) and its binary companion rushing away in the opposite direction?

Ann

My understanding is that it involves the transfer of angular momentum while the stars' Roche lobes overlap during the SN of one of them.

Of course, Chris. I mean - thanks!

Ann

[Chris Peterson]

Do I take that to mean that the star going supernova sheds a lot of mass during its explosion, so that the dramatic weight-loss supernova remnant can't hold on to its companion the way it could before it exploded (when it was a lot more massive)?

Or does it mean, perhaps, that the supernova exploded "asymmetrically", sending perhaps both the compact remnant (if there is one) and its binary companion rushing away in the opposite direction?

Ann

My understanding is that it involves the transfer of angular momentum while the stars' Roche lobes overlap during the SN of one of them.

Of course, Chris. I mean - thanks!

Ann

Even otherwise simple two-body interactions get complicated quickly when you have the mass of one or both bodies changing!

[shaileshs]

I recall somewhere it was mentioned that some star was moving in our direction that is expected to be closer to us than Alpha Centauri is, I'm assuming that star is a runaway star..

[Ann]

I recall somewhere it was mentioned that some star was moving in our direction that is expected to be closer to us than Alpha Centauri is, I'm assuming that star is a runaway star..

No, I don't think so.

Popular Mechanics wrote:

Right now, our sun is fairly isolated. The closest star to our solar system is Alpha Centauri, which is four light-years away—a blink of an eye in cosmic terms, but still extremely distant to us humans.

In 1.3 million years, however, that will all change. Studies by the European Space Agency's Gaia satellite show that a star called Gliese 710 will drop in on our solar system for a brief visit around that time.

Gliese 710 is about 60 percent as massive as the sun. Our new neighbor won't get too close. Rather, it'll linger in the Oort cloud, the icy dust and debris at the outer edges of the solar system. That'll put it 90 light days, or 1.4 trillion miles, away from us, 16,000 times farther from Earth than the sun. But that's close enough to have an impact. At that time, Gliese 710 will shine three times brighter than Mars. More importantly, its gravity could shoot comets and frozen planet into our solar system, putting them on a potential collision course with Earth.

The current distance of Gliese 710 from the solar system is 62 light-years. Its mass is 60 percent the mass of the Sun, which is a really respectable mass when you think of it, but the luminosity of Gliese 710 is only 4 percent of the Sun. The temperature of the photosphere of the Sun - its visible "surface" - is 5778 K, but the temperature of Gliese 710 is "only" 4250 K.

All in all, Gliese 710 is, by human standards, a horribly massive and hot object which is headed our way, although it is very non-remarkable as stars go.

I take this illustration to show large numbers of comets heading towards the inner solar system, because they have been disturbed by Gliese 710 (small red object at upper left). Credit: NASA/JPL-Caltech

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Gliese 710 is not a runaway star. I asked Google about the velocity of Gliese 710, and I got this answer:

Universe Today wrote:

Dr. Bailor-Jones also determined that of the 300,000 stars he observed, 97 of them would pass within 150 trillion km (93 trillion mi; 1 million AU) of our Solar System, while 16 would come within 60 trillion km. While this would be close enough to disturb the Oort Cloud, only one star would get particularly close. That star is Gliese 710, a K-type yellow dwarf located about 63 light years from Earth which is about half the size of our Sun.

According to Dr. Bailer-Jones’ study, this star will pass by our Solar System in 1.3 million years, and at a distance of just 2.3 trillion km (1.4 trillion mi; 16 ,000AU). This will place it well within the Oort Cloud, and will likely turn many icy planetesimals into long-period comets that could head towards Earth. What’s more, Gliese 710 has a relatively slow velocity compared to other stars in our galaxy.

Whereas the average relative velocity of stars is estimated to be around 100.000 km/h (62,000 mph) at their closest approach, Gliese 710 will will have a speed of 50,000 km/h (31,000 mph). As a result, the star will have plenty of time to exert its gravitational influence on the Oort Cloud, which could potentially send many, many comets towards Earth and the inner Solar System.

Ann

[johnnydeep]

So are there sequences of photographs that can show the runaway stars moving through space? Moving apart from each other or nearby objects? Is that how their velocities are calculated? What if they are heading more toward earth or away from it? Can their movements still be measured?

In general, a star is deemed to be a "run-away" if its velocity and/or direction is significantly different from other nearby stars. The velocities of stars are calculated based on their distance and their change in position over time as determined by comparing images from telescopes.

From https://en.wikipedia.org/wiki/Stellar_k ... e_velocity,

Space velocity
The component of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect. The transverse, or proper motion must be found by taking a series of positional determinations against more distant objects. Once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed.[2] This is the star's actual motion relative to the Sun or the local standard of rest (LSR). The latter is typically taken as a position at the Sun's present location that is following a circular orbit around the Galactic Center at the mean velocity of those nearby stars with low velocity dispersion.[3] The Sun's motion with respect to the LSR is called the "peculiar solar motion".

Relation between proper motion and velocity components of an object. At emission, the object was at distance d from the Sun, and moved at angular rate μ radian/s, that is, μ = vt / d with vt = the component of velocity transverse to line of sight from the Sun. (The diagram illustrates an angle μ swept out in unit time at tangential velocity vt.)

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[Chris Peterson]

So are there sequences of photographs that can show the runaway stars moving through space? Moving apart from each other or nearby objects? Is that how their velocities are calculated? What if they are heading more toward earth or away from it? Can their movements still be measured?

In general, a star is deemed to be a "run-away" if its velocity and/or direction is significantly different from other nearby stars. The velocities of stars are calculated based on their distance and their change in position over time as determined by comparing images from telescopes.

From https://en.wikipedia.org/wiki/Stellar_k ... e_velocity,

Space velocity
The component of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect. The transverse, or proper motion must be found by taking a series of positional determinations against more distant objects. Once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed.[2] This is the star's actual motion relative to the Sun or the local standard of rest (LSR). The latter is typically taken as a position at the Sun's present location that is following a circular orbit around the Galactic Center at the mean velocity of those nearby stars with low velocity dispersion.[3] The Sun's motion with respect to the LSR is called the "peculiar solar motion".

Relation between proper motion and velocity components of an object. At emission, the object was at distance d from the Sun, and moved at angular rate μ radian/s, that is, μ = vt / d with vt = the component of velocity transverse to line of sight from the Sun. (The diagram illustrates an angle μ swept out in unit time at tangential velocity vt.)

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It is only recently that we have been able to measure the proper motion for more than a handful of nearby stars. Before Hipparcos and Gaia we simply didn't have the sensitivity to see that motion for most stars. So all we had to work with in most cases was the radial velocity, which is trivially determined spectroscopically from Doppler shift.

[johnnydeep]

So are there sequences of photographs that can show the runaway stars moving through space? Moving apart from each other or nearby objects? Is that how their velocities are calculated? What if they are heading more toward earth or away from it? Can their movements still be measured?

In general, a star is deemed to be a "run-away" if its velocity and/or direction is significantly different from other nearby stars. The velocities of stars are calculated based on their distance and their change in position over time as determined by comparing images from telescopes.

From https://en.wikipedia.org/wiki/Stellar_k ... e_velocity,

Space velocity
The component of stellar motion toward or away from the Sun, known as radial velocity, can be measured from the spectrum shift caused by the Doppler effect. The transverse, or proper motion must be found by taking a series of positional determinations against more distant objects. Once the distance to a star is determined through astrometric means such as parallax, the space velocity can be computed.[2] This is the star's actual motion relative to the Sun or the local standard of rest (LSR). The latter is typically taken as a position at the Sun's present location that is following a circular orbit around the Galactic Center at the mean velocity of those nearby stars with low velocity dispersion.[3] The Sun's motion with respect to the LSR is called the "peculiar solar motion".

Relation between proper motion and velocity components of an object. At emission, the object was at distance d from the Sun, and moved at angular rate μ radian/s, that is, μ = vt / d with vt = the component of velocity transverse to line of sight from the Sun. (The diagram illustrates an angle μ swept out in unit time at tangential velocity vt.)

---

It is only recently that we have been able to measure the proper motion for more than a handful of nearby stars. Before Hipparcos and Gaia we simply didn't have the sensitivity to see that motion for most stars. So all we had to work with in most cases was the radial velocity, which is trivially determined spectroscopically from Doppler shift.

And is just the radial velocity enough to determine whether a star is a run-away?

[Chris Peterson]

In general, a star is deemed to be a "run-away" if its velocity and/or direction is significantly different from other nearby stars. The velocities of stars are calculated based on their distance and their change in position over time as determined by comparing images from telescopes.

From https://en.wikipedia.org/wiki/Stellar_k ... e_velocity,

Relation between proper motion and velocity components of an object. At emission, the object was at distance d from the Sun, and moved at angular rate μ radian/s, that is, μ = vt / d with vt = the component of velocity transverse to line of sight from the Sun. (The diagram illustrates an angle μ swept out in unit time at tangential velocity vt.)

---

It is only recently that we have been able to measure the proper motion for more than a handful of nearby stars. Before Hipparcos and Gaia we simply didn't have the sensitivity to see that motion for most stars. So all we had to work with in most cases was the radial velocity, which is trivially determined spectroscopically from Doppler shift.

And is just the radial velocity enough to determine whether a star is a run-away?

Only if the radial velocity alone is greater than the galactic escape velocity at that star's location (assuming we know the distance... another thing that was hard to figure before Hipparcos and Gaia). Consider the worst case, where the radial motion is zero, but the star is a runaway. Unless we can measure its proper motion, we would never know.

[johnnydeep]

It is only recently that we have been able to measure the proper motion for more than a handful of nearby stars. Before Hipparcos and Gaia we simply didn't have the sensitivity to see that motion for most stars. So all we had to work with in most cases was the radial velocity, which is trivially determined spectroscopically from Doppler shift.

And is just the radial velocity enough to determine whether a star is a run-away?

Only if the radial velocity alone is greater than the galactic escape velocity at that star's location (assuming we know the distance... another thing that was hard to figure before Hipparcos and Gaia). Consider the worst case, where the radial motion is zero, but the star is a runaway. Unless we can measure its proper motion, we would never know.