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












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
    yesterday






  • 1




    $begingroup$
    Another similar question on Astronomy - Why not take a picture of a closer black hole?
    $endgroup$
    – BruceWayne
    15 hours ago


















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
    yesterday






  • 1




    $begingroup$
    Another similar question on Astronomy - Why not take a picture of a closer black hole?
    $endgroup$
    – BruceWayne
    15 hours ago
















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 16 hours ago









Peter Mortensen

1,95511323




1,95511323










asked yesterday









MaxterMaxter

332210




332210








  • 9




    $begingroup$
    Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
    $endgroup$
    – HDE 226868
    yesterday






  • 1




    $begingroup$
    Another similar question on Astronomy - Why not take a picture of a closer black hole?
    $endgroup$
    – BruceWayne
    15 hours ago
















  • 9




    $begingroup$
    Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
    $endgroup$
    – HDE 226868
    yesterday






  • 1




    $begingroup$
    Another similar question on Astronomy - Why not take a picture of a closer black hole?
    $endgroup$
    – BruceWayne
    15 hours ago










9




9




$begingroup$
Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
$endgroup$
– HDE 226868
yesterday




$begingroup$
Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/30313/2153.
$endgroup$
– HDE 226868
yesterday




1




1




$begingroup$
Another similar question on Astronomy - Why not take a picture of a closer black hole?
$endgroup$
– BruceWayne
15 hours ago






$begingroup$
Another similar question on Astronomy - Why not take a picture of a closer black hole?
$endgroup$
– BruceWayne
15 hours ago












1 Answer
1






active

oldest

votes


















43












$begingroup$

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:
$$ tau sim 30 left(frac{M}{10^6 M_{odot}}right) {rm seconds}.$$
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
    yesterday






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    yesterday






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    23 hours ago






  • 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
    13 hours ago






  • 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
    10 hours ago












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43












$begingroup$

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:
$$ tau sim 30 left(frac{M}{10^6 M_{odot}}right) {rm seconds}.$$
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
    yesterday






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    yesterday






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    23 hours ago






  • 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
    13 hours ago






  • 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
    10 hours ago
















43












$begingroup$

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:
$$ tau sim 30 left(frac{M}{10^6 M_{odot}}right) {rm seconds}.$$
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
    yesterday






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    yesterday






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    23 hours ago






  • 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
    13 hours ago






  • 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
    10 hours ago














43












43








43





$begingroup$

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:
$$ tau sim 30 left(frac{M}{10^6 M_{odot}}right) {rm seconds}.$$
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:
$$ tau sim 30 left(frac{M}{10^6 M_{odot}}right) {rm seconds}.$$
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 6 hours ago

























answered yesterday









Rob JeffriesRob Jeffries

71k7150248




71k7150248








  • 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
    yesterday






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    yesterday






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    23 hours ago






  • 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
    13 hours ago






  • 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
    10 hours ago














  • 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
    yesterday






  • 1




    $begingroup$
    I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
    $endgroup$
    – Allure
    yesterday






  • 4




    $begingroup$
    @Allure The centre isn't obscured at 1.3mm wavelengths.
    $endgroup$
    – Rob Jeffries
    23 hours ago






  • 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
    13 hours ago






  • 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
    10 hours ago








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
yesterday




$begingroup$
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
yesterday




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
yesterday




$begingroup$
I'd intuitively think that dust in the disk of our galaxy plays a part by obscuring the innermost regions.
$endgroup$
– Allure
yesterday




4




4




$begingroup$
@Allure The centre isn't obscured at 1.3mm wavelengths.
$endgroup$
– Rob Jeffries
23 hours ago




$begingroup$
@Allure The centre isn't obscured at 1.3mm wavelengths.
$endgroup$
– Rob Jeffries
23 hours ago




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
13 hours ago




$begingroup$
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
13 hours ago




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
10 hours ago




$begingroup$
@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
10 hours ago


















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Liste der Baudenkmäler in Enneberg

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Die Liste der Baudenkmäler in Enneberg (ladinisch Mareo , italienisch Marebbe ) enthält die 36 als Baudenkmäler ausgewiesenen Objekte auf dem Gebiet der Gemeinde Enneberg in Südtirol. Basis ist das im Internet einsehbare offizielle Verzeichnis der Baudenkmäler in Südtirol. Dabei kann es sich beispielsweise um Sakralbauten, Wohnhäuser, Bauernhöfe und Adelsansitze handeln. Die Reihenfolge in dieser Liste orientiert sich an der Bezeichnung, alternativ ist sie auch nach der Adresse oder dem Datum der Unterschutzstellung sortierbar. Liste | Foto   Bezeichnung Standort Eintragung Beschreibung Metadaten ja Altes Gasthaus Janesch ID: 14567 46° 42′ 54″ N, 11° 52′ 42″ O 76882557800 0 8. Sep. 1986 (BLR-LAB 4974) Ländliches Wohnhaus/Bauernhof KG:  Welschellen Bauparzelle: 391 ja Asch (Prack) ID: 14540 46° 43′ 12″ N, 11° 54′ 35″ O 75496485300 20. Nov. 1950 (MD) Ansitz KG:  Enneberg Bauparzelle: 38 ja Biccia mit Stade
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