APOD: M94: A New Perspective (2010 Jan 14)

[APOD Robot]

M94: A New Perspective

Explanation: Beautiful island universe M94 lies a mere 15 million light-years distant in the northern constellation of the hunting dogs, Canes Venatici. A popular target for astronomers the brighter inner part of the face-on spiral galaxy is about 30,000 light-years across. Traditionally, deep images have been interpreted as showing M94's inner spiral region surrounded by a faint, broad ring of stars. But a new multi-wavelength investigation has revealed previously undetected spiral arms sweeping across the outskirts of the galaxy's disk, an outer disk actively engaged in star formation. At optical wavelengths, M94's outer spiral arms are followed in this remarkable discovery image, processed to enhance the outer disk structure. Background galaxies are visible through the faint outer arms, while the three spiky foreground stars are in our own Milky Way galaxy.

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[Benbrilling]

Why do so many APOD pages include expressions like "...a mere 15 million light-years distant..."? What is mere about 15 million light years? Is this a running joke among astronomers? Heck, in my book even a light year is way more than mere. Sure, I know the universe is big, but that's no excuse to make a mockery of the English language.

[jgabany]

Hi, everything's relative. For example, if you consider that the estimated size of the Universe is approximately 150 thousand million light years in diameter (and growing larger at an increasing rate with each passing second), then I believe many would understand why 15 million light years (or .0001 the diameter of the Universe) would be considered relatively close by someone who makes a living studying the heavens. The old adage still rings true: familiarity breeds contempt. :>)

[neufer]

Why do so many APOD pages include expressions like "...a mere 15 million light-years distant..."? What is mere about 15 million light years? Is this a running joke among astronomers? Heck, in my book even a light year is way more than mere.

The 20 million light years gang: M31, M33, M81 & M82, NGC 5128, M83, M94

Galaxies closer than 20 million lyrs projected onto the supergalactic plane:

[emc]

On the subject of mere, I see the APOD author’s use of ‘mere’ to describe enormous distances as colorful use of language. Although, perhaps it is somewhat overused. I have noticed it frequently in the APOD captions. I have to admit, it is a humorous look at an almost unfathomable metric.

Astronomical distances are extremely difficult for me to grasp and I admire those that can. My tiny little brain simply fails to correlate distance of astronomical unit magnitude.

I enjoy the scalable descriptive comparisons like with yesterday’s Spider and the Fly… the name correlates with the relative sizes. And I quickly knew which one was the Fly.

Today’s APOD shows us another of our galactic neighbors… so beautiful and full of mystery. Is there an Earth-like planet located there??? Besides stimulating imaginative space thoughts, it also dredges up frustration in regard to our ability to explore. Our spacecraft can’t reasonably explore much beyond our solar system! While I think it is absolutely incredible what astronomers and cosmologists are teaching us with the array of instruments at their disposal. When it comes to space travel, we are collectively kind of like a very small child looking around at huge quantities of goodies in an incredibly large candy store that has giant size counters and shelves. We can look but not reach. But oh what a candy store! Maybe someday we'll have a ladder to get to those shelves.

[Benbrilling]

Hi, everything's relative. For example, if you consider that the estimated size of the Universe is approximately 150 thousand million light years in diameter (and growing larger at an increasing rate with each passing second), then I believe many would understand why 15 million light years (or .0001 the diameter of the Universe) would be considered relatively close by someone who makes a living studying the heavens. The old adage still rings true: familiarity breeds contempt. :>)

I agree, everything is relative. So if you encounter a neighbor walking down a rural road and ask him how far it is to the nearest inn and he says it is a mere 56,000 km, you might think him a bit daft, or a comedian. Yet that distance is a mere 0.0001 the distance a human of our children's generation ever might hope to travel (minimum distance to Mars.)

[Benbrilling]

Neufer: Thanks for the map! I'm curious: on the map M94 appears to be more like 18 million lyrs away. Is this due to the imprecision of our knowledge of exact distances or did the author of the article round off the distance to 15 million?

[geckzilla]

But the circumference of the earth is less than 56,000 km so that wouldn't be a very funny joke since he was lying to me. Now if he told me it was a mere 5,600,000 millimeters away and it actually was, then I would be amused.

[Benbrilling]

Emc: I share your awe at the size of our universe. And your analogy brings to mind my awe when I first walked into the Bancroft Library at the University of California, Berkeley. Even though it contained just a small selection of the books of the entire University Library, I was so overwhelmed by the rows and rows of books that I felt it was hopeless to even attempt to read a significant fraction of them.

[neufer]

Neufer: Thanks for the map! I'm curious: on the map M94 appears to be more like 18 million lyrs away. Is this due to the imprecision of our knowledge of exact distances or did the author of the article round off the distance to 15 million?

Probably.

Wikipedia has a distance 16.0 ± 1.3 Mly

[Case]

Is this due to the imprecision of our knowledge of exact distances or did the author of the article round off the distance to 15 million?

NED says various papers on M94 estimate the distance between 4.3 and 6.34 megaparsec, with a mean value of 5.125 Mpc (16.7 million lightyear), so there is a significant uncertainty.

[Chris Peterson]

Neufer: Thanks for the map! I'm curious: on the map M94 appears to be more like 18 million lyrs away. Is this due to the imprecision of our knowledge of exact distances or did the author of the article round off the distance to 15 million?

Very few astronomical distances are known with much accuracy. For most distant objects, it is common to see a range of distances that vary by 30% or more. Accurately determining distance is one of the "hard" problems in astronomy.

[Case]

A popular target for astronomers [...] Traditionally, deep images have been interpreted as showing M94's inner spiral region surrounded by a faint, broad ring of stars. [...] Background galaxies are visible through the faint outer arms, while the three spiky foreground stars are in our own Milky Way galaxy.

I had trouble aligning the image with other images of M94; the arms were spiraling in the wrong direction; stars wouldn't match. Until I realized this APOD is vertically flipped, i.e. North and South are switched, but not East and West. I can't think of a reason to do so.
I believe the image is more familiar in this orientation:

[jgabany]

Hello:

You have sharp eyes! The on-line presentation image was flipped for aesthetic reasons.

[WallyWeet]

The perspective, the visual perspective, that haunts me about M94 is that as I gaze on the image I can only see a deep, three dimensional swirling cone moving deeper and deeper down into the heart and center of the image as though the whole thing were a whirlpool taking everything down toward the intensely hot and crowded center. Wrong? But if not what happens as everything crowds into that center point?

[Chris Peterson]

The perspective, the visual perspective, that haunts me about M94 is that as I gaze on the image I can only see a deep, three dimensional swirling cone moving deeper and deeper down into the heart and center of the image as though the whole thing were a whirlpool taking everything down toward the intensely hot and crowded center. Wrong? But if not what happens as everything crowds into that center point?

Bear in mind that "illusion" is the key concept here. This isn't a whirlpool; nothing is moving in towards the center. Each star that is contributing its light to this image is in its own elliptical orbit around the center of the galaxy, moving neither inward nor outward over its lifetime. Spirals draw our eye to their centers, but that doesn't mean anything is actually moving that way. The center of the galaxy is denser than its periphery, but it isn't getting more dense or more crowded with time.