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Big-eyed ostrich, Backwards second rainbow, neutron star’s tiny atmosphere

Q: Which land animal has the largest eyes?  —Jose, Riverside, California

An ostrich has the biggest eyes among land animals. [Corel]

A: An ostrich has the biggest eyes among land animals—2-inch (5 cm) in diameter. A horse has the next biggest, even bigger than elephants’ eyes. All land animal eyes, though, fall woefully short of the biggest eye of all—the 10-inch (25 cm) dinner-plate size giant squid eye. A human’s eye is about an inch across.

Ostriches are extremely wary creatures whose big eyes scan long distances. This, the largest bird, views the land from a small head high on top a long naked neck. When alarmed, she escapes danger—sprinting at 40 miles per hour (64 km/h)

By the way, during courtship, the male sits, opens his wings to show white plumes, rocks back and forth, and twists his neck into a corkscrew.

Q: Why is the second rainbow backwards? —Teddy, Albuquerque, New Mexico

The second bow shines with backwards beauty [NOAA]

A: The second rainbow arches above the first—its red faces the red of the first. The second bow’s colors are "backwards" compared to the first as seen in the figure.

The second bow reverses colors because each sunray enters a raindrop at the bottom (instead of the top of the raindrop) and bounces an extra time (twice) inside the droplet. The figure by Rod Nave shows what happens.

I’m indebted to Rod Nave for this illuminating figure. [Rod Nave, Georgia State University]

Nave’s figure is worth a thousand words in trying to explain why these differences cause red to bend more and therefore ends up lower in the resulting secondary bow. See his figure for the explanation.

Since red bends more than violet, it will likely be too high to hit the viewer's eye—unless, the raindrop is low in the sky.  Consequently, red appears low in the secondary bow.  Likewise, the opposite occurs for a primary bow since red bends less (42°, compared with 40° for violet). See the figure for how red ends up at the top of the primary bow. Thus, the reds of the two bows end up together and the colors throughout are reversed.

The second bow is almost always visible. Though faintly. It’s only one tenth as intense as the primary and almost twice as broad. It’s weaker because less light survives the longer trip in the raindrop and the extra bend.

Further Surfing:

Rod Nave, Hyperphysics: Rainbows

Weather Basics: Rainbows

Q: Re: 11 July neutron star question. What is the star’s atmosphere made of? How can it have a magnetic field since the neutron star has no charge (by definition of a neutron)? —Lanney, Albuquerque, New Mexico and Kevin, Penryn, California

A 15,000 year-old supernova remnant. Neutron stars are supernova corpses (from stars not big enough to die as a black hole). [J.J. Hester, Arizona State University and NASA]

A: A thin wispy cloud of hydrogen and helium probably make up a neutron star’s inches deep atmosphere. Whatever is lightest forms the atmosphere since the star’s tremendous gravity field causes heavy elements to sink quickly. Most of the star is iron but some hydrogen and helium remain from its star-burning days.

"That’s the most realistic possibility," says Coleman Miller, astronomer at the University of Maryland.

We’d like to check spectral lines from an isolated star to know for sure what the atmosphere is. However, "the Catch-22 is that, if the atmosphere is mainly hydrogen and helium, the star temperature is high enough to ionize the gasses completely. We won’t see any lines!"

About the magnetic field—actually, a neutron star does have free charges floating around. It isn’t made up entirely of neutrons. Most, but not all. Even deep in the star interior one proton (positive charge) exists for every ten neutrons. An electron (negative charge) must exist for every proton because the star as a whole must be charge neutral. Otherwise, it would sweep up every charge in the near vicinity until it was charge neutral.

Since the star has free charges, they move and, in so doing, generate a magnetic field. But why such a huge one, you may wonder. Good question. We think the turbulent movement during the star’s collapse may have acted like an enormous magneto-dynamo and generated the monster field. The only problem with this theory is such an effect would generate a much stronger field than even the one we have measured.

"Why is it so much weaker?" Muses Miller. "Beats me!"

Further Surfing:

WonderQuest: Gazing around a neutron star

M. Coleman Miller, University of Maryland: Introduction to neutron stars

(Answered Aug. 15, 2003)