Why was M87 targeted for the Event Horizon Telescope instead of Sagittarius A*? Announcing the arrival of Valued Associate #679: Cesar Manara Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)Why couldn't they take pictures of a closer black hole?Why was the imaging of M87 black hole easier than imaging the Milky way's Sagittarius A*?How is the mass of black hole at the center of our galaxy measured?What happens to the wavelength/frequency of a photon as it passes through an event horizon?Why the center of our galaxy doesn't absorb us?How Can Anything Escape A Supermassive Black Hole?Are black holes in a binary system with white holes, and are they both wormholes?Observer inside event horizon of an extremely large black holeIs it possible the space-time manifold itself could stop at a black hole's event horizon?Picture of Sgr A*First Black Hole Picture TakeawaysWhy couldn't they take pictures of a closer black hole?

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Why was M87 targeted for the Event Horizon Telescope instead of Sagittarius A*?



Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 17/18, 2019 at 00:00UTC (8:00pm US/Eastern)Why couldn't they take pictures of a closer black hole?Why was the imaging of M87 black hole easier than imaging the Milky way's Sagittarius A*?How is the mass of black hole at the center of our galaxy measured?What happens to the wavelength/frequency of a photon as it passes through an event horizon?Why the center of our galaxy doesn't absorb us?How Can Anything Escape A Supermassive Black Hole?Are black holes in a binary system with white holes, and are they both wormholes?Observer inside event horizon of an extremely large black holeIs it possible the space-time manifold itself could stop at a black hole's event horizon?Picture of Sgr A*First Black Hole Picture TakeawaysWhy couldn't they take pictures of a closer black hole?










31












$begingroup$


The first image of a black hole has been released today, April 10th, 2019. The team targeted the black hole at the center of the M87 galaxy.



Why didn't the team target Sagittarius A* at the center of our own galaxy? Intuitively, it would seem to be a better target as it is closer to us.






black-holes astronomy event-horizon







share|cite|improve this question











$endgroup$







  • 9




    $begingroup$
    Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
    $endgroup$
    – HDE 226868
    Apr 10 at 17:29






  • 1




    $begingroup$
    Another similar question on Astronomy - Why not take a picture of a closer black hole?
    $endgroup$
    – BruceWayne
    Apr 11 at 15:07
















31












$begingroup$


The first image of a black hole has been released today, April 10th, 2019. The team targeted the black hole at the center of the M87 galaxy.



Why didn't the team target Sagittarius A* at the center of our own galaxy? Intuitively, it would seem to be a better target as it is closer to us.






black-holes astronomy event-horizon







share|cite|improve this question











$endgroup$







  • 9




    $begingroup$
    Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
    $endgroup$
    – HDE 226868
    Apr 10 at 17:29






  • 1




    $begingroup$
    Another similar question on Astronomy - Why not take a picture of a closer black hole?
    $endgroup$
    – BruceWayne
    Apr 11 at 15:07














31












31








31


5



$begingroup$


The first image of a black hole has been released today, April 10th, 2019. The team targeted the black hole at the center of the M87 galaxy.



Why didn't the team target Sagittarius A* at the center of our own galaxy? Intuitively, it would seem to be a better target as it is closer to us.






black-holes astronomy event-horizon







share|cite|improve this question











$endgroup$




The first image of a black hole has been released today, April 10th, 2019. The team targeted the black hole at the center of the M87 galaxy.



Why didn't the team target Sagittarius A* at the center of our own galaxy? Intuitively, it would seem to be a better target as it is closer to us.






black-holes astronomy event-horizon




black-holes astronomy event-horizon






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Apr 11 at 13:47









Peter Mortensen

1,95511323




1,95511323










asked Apr 10 at 17:24









MaxterMaxter

332210




332210







  • 9




    $begingroup$
    Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
    $endgroup$
    – HDE 226868
    Apr 10 at 17:29






  • 1




    $begingroup$
    Another similar question on Astronomy - Why not take a picture of a closer black hole?
    $endgroup$
    – BruceWayne
    Apr 11 at 15:07













  • 9




    $begingroup$
    Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
    $endgroup$
    – HDE 226868
    Apr 10 at 17:29






  • 1




    $begingroup$
    Another similar question on Astronomy - Why not take a picture of a closer black hole?
    $endgroup$
    – BruceWayne
    Apr 11 at 15:07








9




9




$begingroup$
Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
$endgroup$
– HDE 226868
Apr 10 at 17:29




$begingroup$
Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
$endgroup$
– HDE 226868
Apr 10 at 17:29




1




1




$begingroup$
Another similar question on Astronomy - Why not take a picture of a closer black hole?
$endgroup$
– BruceWayne
Apr 11 at 15:07





$begingroup$
Another similar question on Astronomy - Why not take a picture of a closer black hole?
$endgroup$
– BruceWayne
Apr 11 at 15:07











1 Answer
1






active

oldest

votes


















44












$begingroup$

Of course they targeted Sgr A* as well.



I think though that this is a more difficult target to get good images of.



The black hole is about 1500 times less massive than in M87, but is about 2000 times closer. So the angular scale of the event horizons should be similar. However Sgr A* is a fairly dormant black hole and may not be illuminated so well, and there is more scattering material between us and it than in M87.



A bigger problem may be variability timescales$^dagger$. The black hole in M87 is light days across, so images can be combined across several days of observing. Sgr A* is light minutes across, so rapid variability could be a problem.



The penultimate paragraph of the initial Event Horizon Telescope paper says:




Another primary EHT source, Sgr A*, has a precisely measured mass three orders of magnitude smaller than that of M87*, with dynamical timescales of minutes instead of days. Observing the shadow of Sgr A* will require accounting for this variability and mitigation of scattering effects caused by the interstellar medium




$dagger$ The accretion flow into a black hole is turbulent and variable. However, the shortest timescale upon which significant changes can take place across the source is the timescale for light (the fastest possible means of communication) to travel across or around it. Because the material close to the black hole is moving relativistically, we do expect things to vary on these kinds of timescales. The photon sphere of a black hole is approximately $6GM/c^2$ across, meaning a shortest timescale of variability is about $6GM/c^3$. In more obvious units:
$$ tau sim 30 left(fracM10^6 M_odotright) rm seconds.$$
i.e. We might expect variability in the image on timescales of 30 seconds multiplied by the black hole mass in units of millions of solar masses. This is 2 minutes for Sgr A* and a much longer 2.25 days for the M87 black hole.






share|cite|improve this answer











$endgroup$








  • 4




    $begingroup$
    I was going to protest this answer, but now just have a catch to add. In some places (looking at you, Veritasium) a simulated image of SgrA* is easy to mistake as a genuine photo. Now I understand why SgrA* isn't even in the press release. The circulating SgrA* image is just a simulation. See source material and comments section: youtu.be/VnsZj9RvhFU
    $endgroup$
    – World Outsider
    Apr 10 at 23:09






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    Apr 10 at 23:52






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 6:29






  • 2




    $begingroup$
    So why not Andromeda, or any closer galaxy? Size of central black hole? Orientation of galaxy (edge-on, face-on, or in between)?
    $endgroup$
    – David Conrad
    Apr 11 at 17:01






  • 1




    $begingroup$
    @DavidConrad You will find another question about that somewhere. Yes, the angular size of the Andromeda black hole would be a bit smaller.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 19:23











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1 Answer
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1 Answer
1






active

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active

oldest

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active

oldest

votes









44












$begingroup$

Of course they targeted Sgr A* as well.



I think though that this is a more difficult target to get good images of.



The black hole is about 1500 times less massive than in M87, but is about 2000 times closer. So the angular scale of the event horizons should be similar. However Sgr A* is a fairly dormant black hole and may not be illuminated so well, and there is more scattering material between us and it than in M87.



A bigger problem may be variability timescales$^dagger$. The black hole in M87 is light days across, so images can be combined across several days of observing. Sgr A* is light minutes across, so rapid variability could be a problem.



The penultimate paragraph of the initial Event Horizon Telescope paper says:




Another primary EHT source, Sgr A*, has a precisely measured mass three orders of magnitude smaller than that of M87*, with dynamical timescales of minutes instead of days. Observing the shadow of Sgr A* will require accounting for this variability and mitigation of scattering effects caused by the interstellar medium




$dagger$ The accretion flow into a black hole is turbulent and variable. However, the shortest timescale upon which significant changes can take place across the source is the timescale for light (the fastest possible means of communication) to travel across or around it. Because the material close to the black hole is moving relativistically, we do expect things to vary on these kinds of timescales. The photon sphere of a black hole is approximately $6GM/c^2$ across, meaning a shortest timescale of variability is about $6GM/c^3$. In more obvious units:
$$ tau sim 30 left(fracM10^6 M_odotright) rm seconds.$$
i.e. We might expect variability in the image on timescales of 30 seconds multiplied by the black hole mass in units of millions of solar masses. This is 2 minutes for Sgr A* and a much longer 2.25 days for the M87 black hole.






share|cite|improve this answer











$endgroup$








  • 4




    $begingroup$
    I was going to protest this answer, but now just have a catch to add. In some places (looking at you, Veritasium) a simulated image of SgrA* is easy to mistake as a genuine photo. Now I understand why SgrA* isn't even in the press release. The circulating SgrA* image is just a simulation. See source material and comments section: youtu.be/VnsZj9RvhFU
    $endgroup$
    – World Outsider
    Apr 10 at 23:09






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    Apr 10 at 23:52






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 6:29






  • 2




    $begingroup$
    So why not Andromeda, or any closer galaxy? Size of central black hole? Orientation of galaxy (edge-on, face-on, or in between)?
    $endgroup$
    – David Conrad
    Apr 11 at 17:01






  • 1




    $begingroup$
    @DavidConrad You will find another question about that somewhere. Yes, the angular size of the Andromeda black hole would be a bit smaller.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 19:23















44












$begingroup$

Of course they targeted Sgr A* as well.



I think though that this is a more difficult target to get good images of.



The black hole is about 1500 times less massive than in M87, but is about 2000 times closer. So the angular scale of the event horizons should be similar. However Sgr A* is a fairly dormant black hole and may not be illuminated so well, and there is more scattering material between us and it than in M87.



A bigger problem may be variability timescales$^dagger$. The black hole in M87 is light days across, so images can be combined across several days of observing. Sgr A* is light minutes across, so rapid variability could be a problem.



The penultimate paragraph of the initial Event Horizon Telescope paper says:




Another primary EHT source, Sgr A*, has a precisely measured mass three orders of magnitude smaller than that of M87*, with dynamical timescales of minutes instead of days. Observing the shadow of Sgr A* will require accounting for this variability and mitigation of scattering effects caused by the interstellar medium




$dagger$ The accretion flow into a black hole is turbulent and variable. However, the shortest timescale upon which significant changes can take place across the source is the timescale for light (the fastest possible means of communication) to travel across or around it. Because the material close to the black hole is moving relativistically, we do expect things to vary on these kinds of timescales. The photon sphere of a black hole is approximately $6GM/c^2$ across, meaning a shortest timescale of variability is about $6GM/c^3$. In more obvious units:
$$ tau sim 30 left(fracM10^6 M_odotright) rm seconds.$$
i.e. We might expect variability in the image on timescales of 30 seconds multiplied by the black hole mass in units of millions of solar masses. This is 2 minutes for Sgr A* and a much longer 2.25 days for the M87 black hole.






share|cite|improve this answer











$endgroup$








  • 4




    $begingroup$
    I was going to protest this answer, but now just have a catch to add. In some places (looking at you, Veritasium) a simulated image of SgrA* is easy to mistake as a genuine photo. Now I understand why SgrA* isn't even in the press release. The circulating SgrA* image is just a simulation. See source material and comments section: youtu.be/VnsZj9RvhFU
    $endgroup$
    – World Outsider
    Apr 10 at 23:09






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    Apr 10 at 23:52






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 6:29






  • 2




    $begingroup$
    So why not Andromeda, or any closer galaxy? Size of central black hole? Orientation of galaxy (edge-on, face-on, or in between)?
    $endgroup$
    – David Conrad
    Apr 11 at 17:01






  • 1




    $begingroup$
    @DavidConrad You will find another question about that somewhere. Yes, the angular size of the Andromeda black hole would be a bit smaller.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 19:23













44












44








44





$begingroup$

Of course they targeted Sgr A* as well.



I think though that this is a more difficult target to get good images of.



The black hole is about 1500 times less massive than in M87, but is about 2000 times closer. So the angular scale of the event horizons should be similar. However Sgr A* is a fairly dormant black hole and may not be illuminated so well, and there is more scattering material between us and it than in M87.



A bigger problem may be variability timescales$^dagger$. The black hole in M87 is light days across, so images can be combined across several days of observing. Sgr A* is light minutes across, so rapid variability could be a problem.



The penultimate paragraph of the initial Event Horizon Telescope paper says:




Another primary EHT source, Sgr A*, has a precisely measured mass three orders of magnitude smaller than that of M87*, with dynamical timescales of minutes instead of days. Observing the shadow of Sgr A* will require accounting for this variability and mitigation of scattering effects caused by the interstellar medium




$dagger$ The accretion flow into a black hole is turbulent and variable. However, the shortest timescale upon which significant changes can take place across the source is the timescale for light (the fastest possible means of communication) to travel across or around it. Because the material close to the black hole is moving relativistically, we do expect things to vary on these kinds of timescales. The photon sphere of a black hole is approximately $6GM/c^2$ across, meaning a shortest timescale of variability is about $6GM/c^3$. In more obvious units:
$$ tau sim 30 left(fracM10^6 M_odotright) rm seconds.$$
i.e. We might expect variability in the image on timescales of 30 seconds multiplied by the black hole mass in units of millions of solar masses. This is 2 minutes for Sgr A* and a much longer 2.25 days for the M87 black hole.






share|cite|improve this answer











$endgroup$



Of course they targeted Sgr A* as well.



I think though that this is a more difficult target to get good images of.



The black hole is about 1500 times less massive than in M87, but is about 2000 times closer. So the angular scale of the event horizons should be similar. However Sgr A* is a fairly dormant black hole and may not be illuminated so well, and there is more scattering material between us and it than in M87.



A bigger problem may be variability timescales$^dagger$. The black hole in M87 is light days across, so images can be combined across several days of observing. Sgr A* is light minutes across, so rapid variability could be a problem.



The penultimate paragraph of the initial Event Horizon Telescope paper says:




Another primary EHT source, Sgr A*, has a precisely measured mass three orders of magnitude smaller than that of M87*, with dynamical timescales of minutes instead of days. Observing the shadow of Sgr A* will require accounting for this variability and mitigation of scattering effects caused by the interstellar medium




$dagger$ The accretion flow into a black hole is turbulent and variable. However, the shortest timescale upon which significant changes can take place across the source is the timescale for light (the fastest possible means of communication) to travel across or around it. Because the material close to the black hole is moving relativistically, we do expect things to vary on these kinds of timescales. The photon sphere of a black hole is approximately $6GM/c^2$ across, meaning a shortest timescale of variability is about $6GM/c^3$. In more obvious units:
$$ tau sim 30 left(fracM10^6 M_odotright) rm seconds.$$
i.e. We might expect variability in the image on timescales of 30 seconds multiplied by the black hole mass in units of millions of solar masses. This is 2 minutes for Sgr A* and a much longer 2.25 days for the M87 black hole.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited 2 days ago

























answered Apr 10 at 18:12









Rob JeffriesRob Jeffries

71.4k7152248




71.4k7152248







  • 4




    $begingroup$
    I was going to protest this answer, but now just have a catch to add. In some places (looking at you, Veritasium) a simulated image of SgrA* is easy to mistake as a genuine photo. Now I understand why SgrA* isn't even in the press release. The circulating SgrA* image is just a simulation. See source material and comments section: youtu.be/VnsZj9RvhFU
    $endgroup$
    – World Outsider
    Apr 10 at 23:09






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    Apr 10 at 23:52






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 6:29






  • 2




    $begingroup$
    So why not Andromeda, or any closer galaxy? Size of central black hole? Orientation of galaxy (edge-on, face-on, or in between)?
    $endgroup$
    – David Conrad
    Apr 11 at 17:01






  • 1




    $begingroup$
    @DavidConrad You will find another question about that somewhere. Yes, the angular size of the Andromeda black hole would be a bit smaller.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 19:23












  • 4




    $begingroup$
    I was going to protest this answer, but now just have a catch to add. In some places (looking at you, Veritasium) a simulated image of SgrA* is easy to mistake as a genuine photo. Now I understand why SgrA* isn't even in the press release. The circulating SgrA* image is just a simulation. See source material and comments section: youtu.be/VnsZj9RvhFU
    $endgroup$
    – World Outsider
    Apr 10 at 23:09






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    Apr 10 at 23:52






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 6:29






  • 2




    $begingroup$
    So why not Andromeda, or any closer galaxy? Size of central black hole? Orientation of galaxy (edge-on, face-on, or in between)?
    $endgroup$
    – David Conrad
    Apr 11 at 17:01






  • 1




    $begingroup$
    @DavidConrad You will find another question about that somewhere. Yes, the angular size of the Andromeda black hole would be a bit smaller.
    $endgroup$
    – Rob Jeffries
    Apr 11 at 19:23







4




4




$begingroup$
I was going to protest this answer, but now just have a catch to add. In some places (looking at you, Veritasium) a simulated image of SgrA* is easy to mistake as a genuine photo. Now I understand why SgrA* isn't even in the press release. The circulating SgrA* image is just a simulation. See source material and comments section: youtu.be/VnsZj9RvhFU
$endgroup$
– World Outsider
Apr 10 at 23:09




$begingroup$
I was going to protest this answer, but now just have a catch to add. In some places (looking at you, Veritasium) a simulated image of SgrA* is easy to mistake as a genuine photo. Now I understand why SgrA* isn't even in the press release. The circulating SgrA* image is just a simulation. See source material and comments section: youtu.be/VnsZj9RvhFU
$endgroup$
– World Outsider
Apr 10 at 23:09




1




1




$begingroup$
I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
$endgroup$
– Allure
Apr 10 at 23:52




$begingroup$
I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
$endgroup$
– Allure
Apr 10 at 23:52




4




4




$begingroup$
@Allure The centre isn't obscured at 1.3mm wavelengths.
$endgroup$
– Rob Jeffries
Apr 11 at 6:29




$begingroup$
@Allure The centre isn't obscured at 1.3mm wavelengths.
$endgroup$
– Rob Jeffries
Apr 11 at 6:29




2




2




$begingroup$
So why not Andromeda, or any closer galaxy? Size of central black hole? Orientation of galaxy (edge-on, face-on, or in between)?
$endgroup$
– David Conrad
Apr 11 at 17:01




$begingroup$
So why not Andromeda, or any closer galaxy? Size of central black hole? Orientation of galaxy (edge-on, face-on, or in between)?
$endgroup$
– David Conrad
Apr 11 at 17:01




1




1




$begingroup$
@DavidConrad You will find another question about that somewhere. Yes, the angular size of the Andromeda black hole would be a bit smaller.
$endgroup$
– Rob Jeffries
Apr 11 at 19:23




$begingroup$
@DavidConrad You will find another question about that somewhere. Yes, the angular size of the Andromeda black hole would be a bit smaller.
$endgroup$
– Rob Jeffries
Apr 11 at 19:23

















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