APOD: Trapezium: At the Heart of Orion (2024 Jan 05)

[APOD Robot]

Trapezium: At the Heart of Orion

Explanation: Near the center of this sharp cosmic portrait, at the heart of the Orion Nebula, are four hot, massive stars known as the Trapezium. Gathered within a region about 1.5 light-years in radius, they dominate the core of the dense Orion Nebula Star Cluster. Ultraviolet ionizing radiation from the Trapezium stars, mostly from the brightest star Theta-1 Orionis C powers the complex star forming region's entire visible glow. About three million years old, the Orion Nebula Cluster was even more compact in its younger years and a dynamical study indicates that runaway stellar collisions at an earlier age may have formed a black hole with more than 100 times the mass of the Sun. The presence of a black hole within the cluster could explain the observed high velocities of the Trapezium stars. The Orion Nebula's distance of some 1,500 light-years would make it one of the closest known black holes to planet Earth.

<< Previous APOD

This Day in APOD

Next APOD >>

[Ann]

Trapezium: At the Heart of Orion. Image Credit & Copyright: Fred Zimmer, Telescope Live

---

What I find most interesting about this picture, since I am the Color Commentator after all, is the gray-green color of the nebulosity surrounding the Trapezium, as opposed to the red color farther away from the hot stars. Indeed, I have once observed the Trapezium through a 14 inch telescope, and I vividly remember the gray-green color of the nebulosity surrounding the stars.

In Salvatore Iovene's image below, the Trapezium nebulosity is even greener.

The Trapezium Cluster. Copyright: Salvatore Iovene

---

The green color is caused by OIII emission, which is found close to very hot stars. Farther away from the hottest stars, the nebulosity is dominated by red hydrogen alpha. But our human eyes are not able to see very faint emission of light so close to the long-wave limit of our vision, so we can't see the red color of H-alpha nebulas. However, our eyes are particularly sensitive to green light, which is why it is possible for us to really see the greenish color OIII light.

I guess that most of you are most interested in the possible black hole in the Trapezium. But I'll leave that discussion to the rest of you!

Ann

[Christian G.]

After weeks of cloudy nights where I live, there were finally clear skies last night and I gazed at every part of Orion until exhaustion! So, wonderful to see this APOD today...
(and if the presence of a black hole is confirmed at the heart of the Trapezium, this would be icing on the cake for this glorious region!)

[Tszabeau]

I wonder what it looks like from another angle. Say, if it was rotated 90 degrees horizontal, to our perspective. It appears “cylindrical” from this side. Is that just my projection?

[johnnydeep]

So, it says that Theta-1 Orionis C powers most of the nebula's glow via UV radiation, and that that star is 200,000 times the luminosity of the Sun. But the four Trapezium stars are in a volume 1.5 ly in radius, which implies that almost all the gas that's being ionized must now be many lightyears away, let's say at least 4+ ly. Four lightyears is - conveniently - about 200,000 AU. Is the radiation really strong enough to do that? The UV flux 4 ly from Theta-1 Orion C would be about the same as the UV flux of the Sun at 1 AU. Can the Sun's UV radiation ionize gas near the Earth?

[ (*) EDIT: I blew the conversion: 4 ly is 200,000 AU, not 5 ly! But it doesn't make much of a difference in the posts that follow... ]

[Chris Peterson]

So, it says that Theta-1 Orionis C powers most of the nebula's glow via UV radiation, and that that star is 200,000 times the luminosity of the Sun. But the four Trapezium stars are in a volume 1.5 ly in radius, which implies that almost all the gas that's being ionized must now be many lightyears away, let's say at least 5+ ly. Five lightyears is - conveniently - about 200,000 AU. Is the radiation really strong enough to do that? The UV flux 5 ly from Theta-1 Orion C would be about the same as the UV flux of the Sun at 1 AU. Can the Sun's UV radiation ionize gas near the Earth?

Luminosity is a measure of total radiated EM energy. The Sun, with a temperature of 5,800 K, has a peak output wavelength of 500 nm, and only a tiny fraction of its output in the UV. Theta-1 Orionis C has a temperature of 39,000 K, so a peak output wavelength of 74 nm... extreme UV. Virtually all of its radiant energy is ionizing.

So you can't compare the luminosities this way.

[johnnydeep]

So, it says that Theta-1 Orionis C powers most of the nebula's glow via UV radiation, and that that star is 200,000 times the luminosity of the Sun. But the four Trapezium stars are in a volume 1.5 ly in radius, which implies that almost all the gas that's being ionized must now be many lightyears away, let's say at least 5+ ly. Five lightyears is - conveniently - about 200,000 AU. Is the radiation really strong enough to do that? The UV flux 5 ly from Theta-1 Orion C would be about the same as the UV flux of the Sun at 1 AU. Can the Sun's UV radiation ionize gas near the Earth?

Luminosity is a measure of total radiated EM energy. The Sun, with a temperature of 5,800 K, has a peak output wavelength of 500 nm, and only a tiny fraction of its output in the UV. Theta-1 Orionis C has a temperature of 39,000 K, so a peak output wavelength of 74 nm... extreme UV. Virtually all of its radiant energy is ionizing.

So you can't compare the luminosities this way.

Ah, I knew their spectra would be different, but didn't realize it was THAT different! The result is that there would be much more and much more energetic UV radiation 200,000 AU from Theta- Orionis C than from the Sun at 1 AU? About how much more? Ten times, 100, 1000?

[ EDIT: <sigh> I blew the units here too: I had lys when I meant AUs! ]

[Ann]

So, it says that Theta-1 Orionis C powers most of the nebula's glow via UV radiation, and that that star is 200,000 times the luminosity of the Sun. But the four Trapezium stars are in a volume 1.5 ly in radius, which implies that almost all the gas that's being ionized must now be many lightyears away, let's say at least 5+ ly. Five lightyears is - conveniently - about 200,000 AU. Is the radiation really strong enough to do that? The UV flux 5 ly from Theta-1 Orion C would be about the same as the UV flux of the Sun at 1 AU. Can the Sun's UV radiation ionize gas near the Earth?

Let's take a look at some of the stars in the Antares/Rho Ophiuchi region and consider why their nebulas are the way they are.

Antares/Rho Ophiuchi region. Credit: Danny Flippo.

---

---

The stars are, clockwise from left, Rho Ophiuchi surrounded by a blue nebula, Sigma Scorpii surrounded by a partly red and partly blue nebula, Antares, surrounded by a yellow nebula, and 22 Scorpii, surrounded by a small blue nebula.

Let's look at another picture too, one which shows us an important star south of the Antares/Rho Ophiuchi complex, namely Tau Scorpii surrounded by a red nebula:

Antares/Rho Ophiuchi region plus Tau Scorpii (at bottom right, surrounded by a large pink nebula). Credit: Jakub Korbel.

---

Consider the color of the nebulas and the temperatures of these stars:

1) Rho Ophiuchi: 21,000 K. Blue reflection nebula.

2) Sigma Scorpii: ~ 27,000 K. Faint red emission nebula and blue reflection nebula.

3) Antares: ~3,700 K. Yellow reflection nebula.

4) 22 Scorpii: 19,600 K. Blue reflection nebula.

5) Tau Scorpii: ~30,000 K. Red emission nebula.


Conclusion: Sigma Scorpii is spectral class B1. Stars cooler than Sigma Scorpii are unable to ionize a red emission nebula. But Tau Scorpii, which is spectral class B0.2, is hotter than Sigma Scorpii and definitely able to ionize an (admittedly faint) red emission nebula.

Rho Ophiuchi, 22 Scorpii and most definitely Antares are unable to ionize an emission nebula, because they are not hot enough. Antares is way, way too cool, but Rho Ophiuchi and 22 Scorpii are also too cool.

And the Sun, at 5,772 K, is way too cool, too. It is quite unable to ionize the Earth's atmosphere.

Ann

[Chris Peterson]

So, it says that Theta-1 Orionis C powers most of the nebula's glow via UV radiation, and that that star is 200,000 times the luminosity of the Sun. But the four Trapezium stars are in a volume 1.5 ly in radius, which implies that almost all the gas that's being ionized must now be many lightyears away, let's say at least 5+ ly. Five lightyears is - conveniently - about 200,000 AU. Is the radiation really strong enough to do that? The UV flux 5 ly from Theta-1 Orion C would be about the same as the UV flux of the Sun at 1 AU. Can the Sun's UV radiation ionize gas near the Earth?

Luminosity is a measure of total radiated EM energy. The Sun, with a temperature of 5,800 K, has a peak output wavelength of 500 nm, and only a tiny fraction of its output in the UV. Theta-1 Orionis C has a temperature of 39,000 K, so a peak output wavelength of 74 nm... extreme UV. Virtually all of its radiant energy is ionizing.

So you can't compare the luminosities this way.

Ah, I knew their spectra would be different, but didn't realize it was THAT different! The result is that there would be much more and much more energetic UV radiation 200,000 ly from Theta- Orionis C than from the Sun at 1 ly. About how much more? Ten times, 100, 1000?

Well, looking at the area under the blackbody curves, and setting 200 nm as the threshold for ionizing radiation, effectively 100% of the hot star's energy is ionizing, and about 0.5% of the Sun's is ionizing. So the actual luminosity ratio (for ionizing radiation) should be more like 40,000,000, not 200,000.

[Ann]

So, it says that Theta-1 Orionis C powers most of the nebula's glow via UV radiation, and that that star is 200,000 times the luminosity of the Sun. But the four Trapezium stars are in a volume 1.5 ly in radius, which implies that almost all the gas that's being ionized must now be many lightyears away, let's say at least 5+ ly. Five lightyears is - conveniently - about 200,000 AU. Is the radiation really strong enough to do that? The UV flux 5 ly from Theta-1 Orion C would be about the same as the UV flux of the Sun at 1 AU. Can the Sun's UV radiation ionize gas near the Earth?

Luminosity is a measure of total radiated EM energy. The Sun, with a temperature of 5,800 K, has a peak output wavelength of 500 nm, and only a tiny fraction of its output in the UV. Theta-1 Orionis C has a temperature of 39,000 K, so a peak output wavelength of 74 nm... extreme UV. Virtually all of its radiant energy is ionizing.

So you can't compare the luminosities this way.

Ah, I knew their spectra would be different, but didn't realize it was THAT different! The result is that there would be much more and much more energetic UV radiation 200,000 ly from Theta- Orionis C than from the Sun at 1 ly. About how much more? Ten times, 100, 1000?

I can't tell you, but consider these light curves:

Light curves of a star like Regulus (B7V, 12,000 K), a star like the Sun (G2V, ~6,000 K) and a star like Antares (M1.5Ia,~3,000 K), Credit: I don't know.

---

Spectrum of an O5 star. Credit: Hyperphysics Concept(?)

---

You can see that the emission of the Sun peaks in the blue-green part of the spectrum, and falls off pretty dramatically in the ultraviolet part of the spectrum. Even a star like Regulus (B7V), which peaks in the ultraviolet part of the spectrum, still falls off relatively steeply after its ultraviolet peak. But look at the O5 star. It keeps rising and rising, emitting more and more light, in the ultraviolet part of the spectrum.

According to Wikipedia, the bolometric (total) luminosity of Theta 1 C Orionis is some 204,000 solar luminosities. Since most of the light that Theta 1 C Orionis emits is ultraviolet light, I'd say that Theta 1 C Orionis emits at least 150,000 times more ultraviolet light than the Sun. Perhaps 180,000 times more ultraviolet light than the Sun. Or indeed, 200,000 times more ultraviolet light than the Sun.

Ann

[Ann]

Luminosity is a measure of total radiated EM energy. The Sun, with a temperature of 5,800 K, has a peak output wavelength of 500 nm, and only a tiny fraction of its output in the UV. Theta-1 Orionis C has a temperature of 39,000 K, so a peak output wavelength of 74 nm... extreme UV. Virtually all of its radiant energy is ionizing.

So you can't compare the luminosities this way.

Ah, I knew their spectra would be different, but didn't realize it was THAT different! The result is that there would be much more and much more energetic UV radiation 200,000 ly from Theta- Orionis C than from the Sun at 1 ly. About how much more? Ten times, 100, 1000?

Well, looking at the area under the blackbody curves, and setting 200 nm as the threshold for ionizing radiation, effectively 100% of the hot star's energy is ionizing, and about 0.5% of the Sun's is ionizing. So the actual luminosity ratio (for ionizing radiation) should be more like 40,000,000, not 200,000.

You beat me to it, Chris. Of course, I didn't know how to figure out how much more ultraviolet light an O6 type star emits compared to the Sun. I guess you told me here.

Disregard my previous post, Johnny.

Ann

[johnnydeep]

Ah, I knew their spectra would be different, but didn't realize it was THAT different! The result is that there would be much more and much more energetic UV radiation 200,000 AU from Theta- Orionis C than from the Sun at 1 AU. About how much more? Ten times, 100, 1000?

Well, looking at the area under the blackbody curves, and setting 200 nm as the threshold for ionizing radiation, effectively 100% of the hot star's energy is ionizing, and about 0.5% of the Sun's is ionizing. So the actual luminosity ratio (for ionizing radiation) should be more like 40,000,000, not 200,000.

You beat me to it, Chris. Of course, I didn't know how to figure out how much more ultraviolet light an O6 type star emits compared to the Sun. I guess you told me here.

Disregard my previous post, Johnny.

Ann

Yeah, so using Chris' numbers, the UV/ionizing radiation from Theta-Orionis C at 4 ly would be about 200 times greater than the UV from the Sun at 1 AU. And I suppose 200 times the UV from the Sun at 1 AU would be enough to ionize any gas there.

[Chris Peterson]

And I suppose 200 times the UV from the Sun at 1 AU would be enough to ionize any gas there.

Sunscreens would have to use scientific notation to advertise their SPF levels!

[Ann]

Well, looking at the area under the blackbody curves, and setting 200 nm as the threshold for ionizing radiation, effectively 100% of the hot star's energy is ionizing, and about 0.5% of the Sun's is ionizing. So the actual luminosity ratio (for ionizing radiation) should be more like 40,000,000, not 200,000.

You beat me to it, Chris. Of course, I didn't know how to figure out how much more ultraviolet light an O6 type star emits compared to the Sun. I guess you told me here.

Disregard my previous post, Johnny.

Ann

Yeah, so using Chris' numbers, the UV/ionizing radiation from Theta-Orionis C at 4 ly would be about 200 times greater than the UV from the Sun at 1 AU. And I suppose 200 times the UV from the Sun at 1 AU would be enough to ionize any gas there.

I still think you underestimate the power of an O6 or O7 type star as Theta 1 C Orionis, Johnny. 4 ly is very close for an O-type star. According to Wikipedia, the size of the Trapezium region is some 10 light-years, and the entire region is certainly very highly ionized. In order to get ionization like that from the Sun, I guess you would have to be inside the corona!

The pink stuff around the Sun's limb is indeed ionized hydrogen. But that's how close you need to be for the Sun to do any significant ionizing. Credit: Luc Viatour

---

Ann

[Chris Peterson]

You beat me to it, Chris. Of course, I didn't know how to figure out how much more ultraviolet light an O6 type star emits compared to the Sun. I guess you told me here.

Disregard my previous post, Johnny.

Ann

Yeah, so using Chris' numbers, the UV/ionizing radiation from Theta-Orionis C at 4 ly would be about 200 times greater than the UV from the Sun at 1 AU. And I suppose 200 times the UV from the Sun at 1 AU would be enough to ionize any gas there.

I still think you underestimate the power of an O6 or O7 type star as Theta 1 C Orionis, Johnny. 4 ly is very close for an O-type star. According to Wikipedia, the size of the Trapezium region is some 10 light-years, and the entire region is certainly very highly ionized. In order to get ionization like that from the Sun, I guess you would have to be inside the corona!

The pink stuff around the Sun's limb is indeed ionized hydrogen. But that's how close you need to be for the Sun to do any significant ionizing. Credit: Luc Viatour

---

Ann

I don't think that's an accurate way of looking at it. The ionized hydrogen we see on the Sun's surface, and being lifted off of it by magnetic fields, is not the same as the ionized hydrogen we see in nebulas. The hydrogen at the Sun's surface is much higher density and is ionized by kinetic heating, not by absorbing UV photons the way that nebular hydrogen is ionized.

[johnnydeep]

And I suppose 200 times the UV from the Sun at 1 AU would be enough to ionize any gas there.

Sunscreens would have to use scientific notation to advertise their SPF levels!

[shaileshs]

Slightly tangential question - if there's BH there (around 1500 ly), then it'd be even closer than the closest we know so far (Gaia BH1/BH2). I wonder if there'd be a BH much closer (e.g. just outside Neptune's orbit) and we just don't have a way (technology, instruments, skills, knowledge) to detect it ? And, one day we may discover some such and it'd be much easier to send probes to it to study it better (from close or even jumping inside event horizon) to understand BH properties a bit better..

[johnnydeep]

Slightly tangential question - if there's BH there (around 1500 ly), then it'd be even closer than the closest we know so far (Gaia BH1/BH2). I wonder if there'd be a BH much closer (e.g. just outside Neptune's orbit) and we just don't have a way (technology, instruments, skills, knowledge) to detect it ? And, one day we may discover some such and it'd be much easier to send probes to it to study it better (from close or even jumping inside event horizon) to understand BH properties a bit better..

There might be a way to detect it via gravitational bending of light from background stars, but Chris could say more.

As for nearby black holes, there might even be (but probably isn't) one orbiting the core of the Sun, inside it! See https://phys.org/news/2023-12-black-hole-sun-1.html

"Stars harboring a black hole at their center can live surprisingly long," says Earl Patrick Bellinger, MPA Postdoc and now Assistant Professor at Yale University, who led the study. "Our sun could even have a black hole as massive at the planet Mercury at its center without us noticing."

[Chris Peterson]

Slightly tangential question - if there's BH there (around 1500 ly), then it'd be even closer than the closest we know so far (Gaia BH1/BH2). I wonder if there'd be a BH much closer (e.g. just outside Neptune's orbit) and we just don't have a way (technology, instruments, skills, knowledge) to detect it ? And, one day we may discover some such and it'd be much easier to send probes to it to study it better (from close or even jumping inside event horizon) to understand BH properties a bit better..

A BH that close to the Solar System would mass more than the Sun. We wouldn't have any planets left in the system.

[johnnydeep]

Slightly tangential question - if there's BH there (around 1500 ly), then it'd be even closer than the closest we know so far (Gaia BH1/BH2). I wonder if there'd be a BH much closer (e.g. just outside Neptune's orbit) and we just don't have a way (technology, instruments, skills, knowledge) to detect it ? And, one day we may discover some such and it'd be much easier to send probes to it to study it better (from close or even jumping inside event horizon) to understand BH properties a bit better..

A BH that close to the Solar System would mass more than the Sun. We wouldn't have any planets left in the system.

But what if it only had a mass of, say, Mercury?

[Chris Peterson]

Slightly tangential question - if there's BH there (around 1500 ly), then it'd be even closer than the closest we know so far (Gaia BH1/BH2). I wonder if there'd be a BH much closer (e.g. just outside Neptune's orbit) and we just don't have a way (technology, instruments, skills, knowledge) to detect it ? And, one day we may discover some such and it'd be much easier to send probes to it to study it better (from close or even jumping inside event horizon) to understand BH properties a bit better..

A BH that close to the Solar System would mass more than the Sun. We wouldn't have any planets left in the system.

But what if it only had a mass of, say, Mercury?

While there is some (largely untested) theory regarding the possibility of tiny primordial black holes, created in the earliest stages of the evolution of the Universe, there is no known mechanism for generating black holes with a mass less than about three suns.

[johnnydeep]

A BH that close to the Solar System would mass more than the Sun. We wouldn't have any planets left in the system.

But what if it only had a mass of, say, Mercury?

While there is some (largely untested) theory regarding the possibility of tiny primordial black holes, created in the earliest stages of the evolution of the Universe, there is no known mechanism for generating black holes with a mass less than about three suns.

Ok. Why does it seem that all the really cool ideas are prohibited (or at least wildly unlikely) given the currently known mechanisms?

PS - The novel Dragon's Egg by Robert L. Forward has a plot point involving a few small black holes inside the Sun that are removed by the helpful Cheela!

[Chris Peterson]

But what if it only had a mass of, say, Mercury?

While there is some (largely untested) theory regarding the possibility of tiny primordial black holes, created in the earliest stages of the evolution of the Universe, there is no known mechanism for generating black holes with a mass less than about three suns.

Ok. Why does it seem that all the really cool ideas are prohibited (or at least wildly unlikely) given the currently known mechanisms?

PS - The novel Dragon's Egg by Robert L. Forward has a plot point involving a few small black holes inside the Sun that are removed by the helpful Cheela!

Good science fiction that incorporates small black holes normally treats them as products of advanced technology. There is no fundamental limit on what mass a black hole can be, it's just that we don't know of any natural process that isn't driven by gravitational collapse, and that requires a few solar masses.

[johnnydeep]

While there is some (largely untested) theory regarding the possibility of tiny primordial black holes, created in the earliest stages of the evolution of the Universe, there is no known mechanism for generating black holes with a mass less than about three suns.

Ok. Why does it seem that all the really cool ideas are prohibited (or at least wildly unlikely) given the currently known mechanisms?

PS - The novel Dragon's Egg by Robert L. Forward has a plot point involving a few small black holes inside the Sun that are removed by the helpful Cheela!

Good science fiction that incorporates small black holes normally treats them as products of advanced technology. There is no fundamental limit on what mass a black hole can be, it's just that we don't know of any natural process that isn't driven by gravitational collapse, and that requires a few solar masses.

Got it. Dragon's Egg is a great book by the way. I recommend it if you haven't already read it.