APOD: LL Orionis: When Cosmic Winds Collide (2016 May 22)

[APOD Robot]

LL Orionis: When Cosmic Winds Collide

Explanation: What created this great arc in space? This arcing, graceful structure is actually a bow shock about half a light-year across, created as the wind from young star LL Orionis collides with the Orion Nebula flow. Adrift in Orion's stellar nursery and still in its formative years, variable star LL Orionis produces a wind more energetic than the wind from our own middle-aged sun. As the fast stellar wind runs into slow moving gas a shock front is formed, analogous to the bow wave of a boat moving through water or a plane traveling at supersonic speed. The slower gas is flowing away from the Orion Nebula's hot central star cluster, the Trapezium, located off the lower right hand edge of the picture. In three dimensions, LL Ori's wrap-around shock front is shaped like a bowl that appears brightest when viewed along the "bottom" edge. The complex stellar nursery in Orion shows a myriad of similar fluid shapes associated with star formation, including the bow shock surrounding a faint star at the upper right. Part of a mosaic covering the Great Nebula in Orion, this composite color image was recorded in 1995 by the Hubble Space Telescope.

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[Boomer12k]

Very nice...

:---[===] *

[BillLee]

The description says "analogous to the bow wave of a boat moving through water or a plane traveling at supersonic speed."

A plane need not be traveling at "supersonic speed" to create a bow wave.

[Chris Peterson]

The description says "analogous to the bow wave of a boat moving through water or a plane traveling at supersonic speed."

A plane need not be traveling at "supersonic speed" to create a bow wave.

That's true. This is not a bow wave, however, but a bow shock, which requires supersonic motion. So in reality, while this shock is somewhat analogous to a bow wave, it is a much more similar phenomenon to the bow shock of a supersonic plane (the only real difference being whether the shock front depends on aerodynamics or a magnetic field).

[starsurfer]

A very shocking image today! It's always cool to see closeups of larger nebulae. A challenge (not specifically for Ann only ), try finding this bowshock in this image of the whole of the Orion Nebula by CHART32. A helpful clue is that the Hubble image is upside down.

[neufer]

The description says "analogous to the bow wave of a boat moving through water or a plane traveling at supersonic speed."

A plane need not be traveling at "supersonic speed" to create a bow wave.

[b][color=#0000FF][size=85]Detached bow wave/shock for a 'supersonic' (>23.1 cm/s) swimmer[/size][/color][/b]

---

That's true. This is not a bow wave, however, but a bow shock, which requires supersonic motion. So in reality, while this shock is somewhat analogous to a bow wave, it is a much more similar phenomenon to the bow shock of a supersonic plane.

A subsonic plane should not have a detached bow wave/shock in front of the plane's nose.

A boat or swimmer that moves faster than the slowest moving water wave (23.1 cm/s) will be 'supersonic' for those (gravity/capillary) waves and hence produce a detached bow wave/shock.

[b][color=#0000FF][size=150]Detached bow wave/shock for a 'supersonic' bullet[/size][/color][/b]

---

<<A bow shock, also called a detached shock or normal shock, is a curved, stationary shock wave that is found in a supersonic flow past a finite body. The name comes from the example of a bow wave that forms at the bow of a ship when it moves through the water.

Unlike an oblique shock, the bow shock is not necessarily attached to the tip of the body. Oblique shock angles are limited in formation and are based on the flow deflection angle, upstream Mach number. When these limitations are exceeded (greater deflection angle or lower Mach number), a detached bow shock forms instead of an oblique shock. As bow shocks form for high flow deflection angles, they are often seen forming around blunt objects. In other words, when the needed rotation of the fluid exceeds the maximum achievable rotation angle for an oblique attached shock, the shock detaches from the body. Downstream of the shock, the flow-field is subsonic, and the boundary condition can be respected at the stagnation point.

The bow shock significantly increases the drag in a vehicle traveling at a supersonic speed. This property was utilized in the design of the return capsules during space missions such as the Apollo program, which need a high amount of drag in order to slow down during atmospheric reentry.>>

[Ann]

A very shocking image today! It's always cool to see closeups of larger nebulae. A challenge (not specifically for Ann only ), try finding this bowshock in this image of the whole of the Orion Nebula by CHART32. A helpful clue is that the Hubble image is upside down.

Well, I found it. In the black and white image, the star and bow shock in today's APOD is

in the lower right corner

and in the color image, it is

down to the lower right from the Trapezium; the star looks yellowish and has no diffraction spikes, but to its right is a greenish star with diffraction spikes and a small yellow star growing like a wart on its upper left.

Ann

[robin egg]

I'm tired of seeing this in APOD. Myriad means many. Do you say 'a many of'? No. Then don't say "a myriad of".

[geckzilla]

I'm tired of seeing this in APOD. Myriad means many. Do you say 'a many of'? No. Then don't say "a myriad of".

Myriad is both a collective noun something like flock and an adjective. In this case, a synonym would be great amount and not many. We quite often do say A great amount of.

[Chris Peterson]

I'm tired of seeing this in APOD. Myriad means many. Do you say 'a many of'? No. Then don't say "a myriad of".

It's a noun, properly used with an article like "a", or pluralized (myriads). And it's been used that way by some of the greatest English writers for at least 500 years. In other words, it's standard English usage, perfectly reasonable and correct.

[neufer]

I'm tired of seeing this in APOD. Myriad means many.

Do you say 'a many of'? No. Then don't say "a myriad of".

It's a noun, properly used with an article like "a", or pluralized (myriads). And it's been used that way by some of the greatest English writers for at least 500 years. In other words, it's standard English usage, perfectly reasonable and correct.

• Perhaps "robin egg" is British.

<<A myriad (from Ancient Greek μυριάς, myrias) is technically the number ten thousand.

In English, "myriad" is most commonly used to mean "some large but unspecified number". It may be either an adjective or a noun: both "there are myriad people outside" and "there is a myriad of people outside" are in use. (There are small differences: the former could imply that it is a diverse group of people; the latter does not but could possibly indicate a group of exactly ten thousand.) The Merriam-Webster Dictionary notes that confusion over the use of myriad as a noun "seems to reflect a mistaken belief that the word was originally and is still properly only an adjective ... however, the noun is in fact the older form, dating to the 16th century. The noun 'myriad' has appeared in the works of such writers as Milton (plural 'myriads') and Thoreau ('a myriad of'), and it continues to occur frequently in reputable English.">>

[Chris Peterson]

I'm tired of seeing this in APOD. Myriad means many.

Do you say 'a many of'? No. Then don't say "a myriad of".

It's a noun, properly used with an article like "a", or pluralized (myriads). And it's been used that way by some of the greatest English writers for at least 500 years. In other words, it's standard English usage, perfectly reasonable and correct.

• Perhaps "robin egg" is British.

You mean like Milton?

[neufer]

Perhaps "robin egg" is British.

You mean like Milton?

Probably not.

I think I prefer the adjective "myriad" here ...
or "myriads of" as in "thousands of" or "millions of".

It is clearer, more traditional, less awkward to say and more succinct than "a myriad of".

<<When myriad first appeared in English, it was always plural and followed by of, such as many myriads of men. Then, in 1609, the singular form of myriad was first used, followed again by of. This allowed for phrases like a myriad of bubbles. Finally, in the 18th century, the noun was first dropped from the phrase. At that time, the saying myriad beauties was then considered correct.

Today, both phrases are used. Although myriad of is a bit more common than myriad followed by a noun, either expression can be used.>>

[Chris Peterson]

I think I prefer the adjective "myriad" here ...
or "myriads of" as in "thousands of" or "millions of".

It is clearer, more traditional, less awkward to say and more succinct than "a myriad of".

I'm content to let the language move where it will. Mainly, I look for clarity and a lack of ambiguity, so any of the "myriad" usages works okay for me.

[starsurfer]

A very shocking image today! It's always cool to see closeups of larger nebulae. A challenge (not specifically for Ann only ), try finding this bowshock in this image of the whole of the Orion Nebula by CHART32. A helpful clue is that the Hubble image is upside down.

Well, I found it. In the black and white image, the star and bow shock in today's APOD is

in the lower right corner

and in the color image, it is

down to the lower right from the Trapezium; the star looks yellowish and has no diffraction spikes, but to its right is a greenish star with diffraction spikes and a small yellow star growing like a wart on its upper left.

Ann

Well done Ann! You win this challenge!