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Whenever there's a star in an image taken by the Hubble Space Telescope, that star has four points or spikes (Source: ESA/NASA/E. Olszewski (University of Arizona))

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Stars look pointy because …

Tuesday 16th June 2015 9:00 am

Whether it’s the star we plonk onto the Christmas tree, the stars that adorn our flags or pyjamas, or the stars on the Walk of Fame — they all have points!

But we all know that a real star doesn’t actually have any points or spikes. A star is a giant spherical ball of plasma. Furthermore, all the stars that we can see (apart from our Sun) are so far away that they appear to us as perfect little dots.

The answer to why we draw stars as pointy objects, is because our eyes actually see them as having points.

And why? Because the lens inside each human eyeball has two imperfections called ‘suture lines’.

Let’s start with some anatomy.

A human eyeball is about the size of a golf ball — 25 millimetres or so. At the very front is the cornea. It does about two-thirds of the bending of incoming light. This light needs to be precisely bent so that it lands exactly on the retina — not in front, nor behind.

Anatomy of the eye

Anatomy of the eye(Source: BruceBlaus/Wikimedia Commons)

The lens does the remaining one third of the bending of the light. The lens has an optical power of 18 dioptres.

This lens inside your eyeball looks like an asymmetrical flattened ball — it’s more flattened at the front, more rounded at the rear. Side-to-side, the lens is about 10 millimetres in diameter, but front-to-back it’s about 4 millimetres. The widest part is called the equator.

Thanks to little muscles attached near the equator, the lens can change its shape. It can do this in one third of a second. So as you look at distant, and then near, objects, the lens changes shape to ensure that the image always lands exactly on the retina.

But it’s only when you are very young, that your lens can change its shape a lot, and vary its optical power by up to 15 of its 18 dioptres.

By the time you reach 45 – 50 years of age, your range of adjustment is down to only two dioptres. If you want to look at both distant and near objects sharply, you need glasses. By the time you reach 70, all you get is one lousy dioptre.

Anatomy of the lens

Anatomy of the lens (Source: Adapted from Openi)

Your lens is almost totally transparent. It has three main parts.

First, the lens capsule wraps around the lens, and holds it together. This membrane is a bit like a plastic bag. It’s made from proteins and sugars. The lens capsule is incredibly thin. It varies between two and 28 microns in thickness (by comparison, a fibre of hair is about 60 microns thick).

The second part is the lens epithelium. It’s a thin layer of simple cuboidal cells, located at the front half of the lens. This layer of cells reaches back to the equator (or widest part) of the lens. It regulates the volume and saltiness of the lens. At the equator, its cells also make new lens fibres that migrate toward the centre-line of the lens. (If you want to learn more, look up ‘lens morphogenesis’.)

And third, there are lens fibres that make up the bulk of the lens. They are actually transparent cells that are both long (up to 12 millimetres) and skinny (3 to 10 microns in diameter).

These fibres meet and blend at Y-shaped suture lines. There are two of these suture lines. The front one is a regular Y-shape, while the back one is an upside-down Y-shape.

These suture lines bend the waves of light as they travel through, and past, them.

This bending is called ‘diffraction’. Whenever light passes either around an object, or through a slit inside an object, it bends and changes its direction.

Now here’s an example of pointy stars you would have seen dozens of times, and perhaps never noticed.

You’ve heard of the famous Hubble Space Telescope, which was launched in 1990. Whenever there’s a star in the image, that star has four points or spikes — even though it’s a round little dot. Why?

Because in the Hubble Space Telescope, the smaller secondary mirror is held in position by four cross hair-like struts, and the incoming light has to travel past these struts to land on the bigger main mirror. This light gets bent, giving the star its characteristic four points.

A similar thing happens with your eyes, thanks to the suture lines inside the lens of your eyeball bending the light. But, because they are ‘organic’, every human eye has slightly different suture lines. Even your right and left eyes have different suture lines — so you might see four points with one eye, but five with the other.

But the point of it all is — there’s nothing like being dazzled by a little starlight …


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This blog first appeared on Dr Karl's Great Moments in Science

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