天文学と天体物理学 101: 赤色巨星

NASA/ESA ハッブル宇宙望遠鏡からのこの異常な画像では、明るい星が希薄なガスの殻に囲まれています。 U Camelopardalis、または略して U Cam は、その寿命の終わりに近い星です。 燃料が少なくなり始めると、不安定になってきます。 数千年ごとに、核の周りのヘリウムの層が融合し始めると、ほぼ球形のガスの殻を吐き出します。 星の最近の噴火で放出されたガスは、星を取り囲むかすかなガスの泡としてこの写真ではっきりと見えます。 クレジット: ESA/ハッブル、NASA、H. オロフソン (オンサラ宇宙天文台)

赤色巨星とは?

赤色巨星は、星が核融合のための水素燃料を使い果たし、死に至るプロセスを開始した後に形成されます。

星は、星を引き寄せる自身の重力と、中心部で進行中の熱核融合プロセスからの外向きの圧力との間の微妙なバランスによって、その安定性を維持しています。

しかし、星のコアが水素を使い果たすと、その平衡状態が失われ、コアが崩壊し始めます。 コアが崩壊するにつれて、シェル[{” attribute=””>plasma surrounding the core becomes hot enough to begin fusing hydrogen itself. As fusion in this shell begins, the extra heat causes the outer layers of the star to greatly expand, and the surface extends up to several hundred times beyond the former size of the star. As the energy at the star’s surface becomes far more dissipated, it causes the star’s bloated surface to cool, turning from white or yellow to red. A red giant is formed.

This process can take hundreds of millions of years to transpire. It only applies to intermediate-mass stars (with a mass between 80% and 800% of the Sun’s mass), which then go on to form planetary nebulae. When a more massive star runs out of hydrogen at its core, it forms a red supergiant instead, and then goes on to explode as a supernova.

赤色巨星は、星が核融合のための水素燃料を使い果たした後に形成され、死に至るプロセスを開始します。 クレジット:[{” attribute=””>NASA & ESA

Red giant stars are used by Hubble to calculate the distances to different galaxies. Astronomers are able to determine how far away galaxies are by comparing the brightness of the galaxies’ red giant stars with nearby red giants, whose distances have been measured by other methods. This is possible because red giants are reliable milepost markers — they all reach the same peak brightness in their late evolution — and so they can be used as a “standard candle” to calculate distance. Hubble’s outstanding sharpness and sensitivity allow it to find red giants in the stellar halos of the galaxies.

Hubble has observed U Camelopardalis, which coughs out a nearly spherical shell of gas as a layer of helium around its core begins to fuse every few thousand years. The shell of gas, which is both much larger and much fainter than its parent star, is visible in intricate detail due to Hubble’s sensitivity. While phenomena that occur at the ends of stars’ lives are often quite irregular and unstable, the telescope has observed that the shell of gas expelled from this red giant is almost perfectly spherical.

Word Bank Red Giant

Credit: ESA/Hubble, NASA and H. Olofsson (Onsala Space Observatory)

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Hubble has also shown us the beautiful and colorful remnants of former red giants. These include NGC 2371, NGC 2022, and NGC 5307.

Hubble’s 29th-anniversary image was of the hourglass-shaped Southern Crab Nebula, which was formed by the interaction between a red giant and a white dwarf. The red giant is shedding its outer layers in the last phase of its life before it too lives out its final years as a

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