사용자:Lee Donghyeon

소개

KSA 학생입니다.

번역 중 : Hubble Sequence (허블의 은하 분류)

허블 순차 혹은 허블 연속(Hubble sequence)은 1926년 에드윈 허블이 제안한 은하의 형태학적 분류로, 오늘날 가장 널리 사용된다. 보통 소리굽쇠 모양의 도표로 분류를 나타낸다.

소리굽쇠 도표

은하의 분류

허블은 은하를 외형에 따라 타원 은하, 렌즈형 은하, 나선 은하로 분류하였으며 어느 곳에도 속하지 않는 경우에는 불규칙 은하에 포함시켰다.

타원 은하

 타원 은하 문서를 참고하십시오.

거대 타원 은하 ESO 325-G004

소리굽쇠 도표의 왼쪽에 위치해 있으며 타원을 의미하는 영단어 Ellipse의 첫글자를 따서 E라고 표현한다. 타원 은하는 편평도에 따라 E0, E1, ... , E6, E7 으로 세분되는데 뒤에 붙는 정수는 편평도의 10배 값을 가장 가까운 정수까지 반올림한 것이다. 편평도는 아래와 같이 정의한다.

편평도 ${\begin{matrix}e=1-{\frac {b}{a}}\end{matrix}}$ (a : 타원의 장반경, b : 은하의 단반경)

편평도가 작아서 원에 가까운 은하일수록 소리굽쇠 그림의 왼쪽에 위치해 있으며 편평도가 클수록 오른쪽에 있다. 중요한 것은 측정한 편평도의 값이 실제의 값과 다를 수 있다는 것이다. 예를 들어 은하가 매우 찌그러진 타원체임에도 불구하고 보는 방향에 따라 원에 가깝게 보일 수 있는 것이다. 대표적인 타원 은하로는 M49, M59, M60, M87, NGC 4125 등이 있다.

나선 은하

 나선 은하 문서를 참고하십시오.

소용돌이 은하 (M 101/NGC 5457): Scd

NGC 1300 : SBbc

소리굽쇠 도표의 오른쪽에는 나선 은하가 위치해 있다. 나선 은하는 중앙팽대부의 형태에 따라 크게 두 종류로 나뉘는데, 하나는 팽대부가 원형인 정상 나선 은하(S) 이고, 나머지는 팽대부가 막대 모양인 막대 나선 은하(SB)이며 도표 상에서 독립된 가지를 차지하고 있다. 두 나선 은하 모두 나선팔의 감긴 정도와 중앙팽대부의 밝기에 따라 세 종류로 세분된다.

• Sa (SBa) - 나선팔이 빡빡하게 감겨있으며 중앙팽대부가 크고 밝다.
• Sb (SBb) - 나선팔이 Sa(SBa)에 비해서는 느슨하게 감겨있으며 중앙팽대부가 조금 어둡고 작다.
• Sc (SBc) - 나선팔이 매우 느슨하게 감겨있으며 성단과 성운을 또렷하게 구분하여 볼 수 있다. 중앙팽대부는 작고 어둡다.

허블이 처음 분류를 제안했을 때는 a, b, c 밖에 없었으나 이후 드 보쿨레르가 소분류 d를 추가하였다.

• Sd (SBd) - 나선팔이 완전히 풀려있으며 중앙팽대부가 아주 작고 어둡다. fragmentary arms

두 분류의 중간 형태를 띠는 은하들은 분류 두 개를 중복 표기하여 나타내기도 한다. 예를 들어 Sbc 은하는 Sb와 Sc의 중간 모습을 지닌 은하다.

비교적 최근까지 우리 은하는 정상 나선 은하로 알려졌으나 최근의 연구 결과에 의하면 막대 나선 은하 SBb인 것으로 밝혀졌다. 그러나 우리가 은하의 바깥쪽에서 관찰한 것이 아닌 탓에 정확한 분류 상 위치에 대해서는 논란이 있다.

주요한 정상 나선 은하로는 M31(안드로메다 은하), M51(소용돌이 은하), M74, M81, NGC 300, NGC 772 등을 들 수 있다. 주요한 막대 나선 은하로는 M91, M95, NGC 1097, NGC 1300, NGC1672, NGC 2536, NGC 2903 등을 들 수 있다.

렌즈형 은하

 렌즈형 은하 문서를 참고하십시오.

The NGC 5866(Spindle Galaxy) (NGC 5866), a lenticular galaxy with a prominent dust lane in the constellation of Draco.

소리굽쇠 도표의 타원 은하 계열과 나선 은하 계열이 만나는 지점에 놓여있다. S0로 표현되는 이 은하는 매우 밝은 중앙팽대부가 있으며 At the centre of the Hubble tuning fork, where the two spiral arms meet the elliptical branch lies an intermediate class of galaxies known as lenticulars and given the symbol S0. These galaxies consist of a bright central bulge, similar in appearance to an elliptical galaxy, surrounded by an extended, disk-like structure. Unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity. The bulge component is often the dominant source of light in a lenticular galaxy.

Face-on lenticulars are difficult to distinguish from ellipticals of type E0, making the classification of many such galaxies uncertain. When viewed edge-on, prominent dust-lanes are sometimes visible in absorption against the light of stars in the disk.

At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxies was purely hypothetical. Hubble believed that they were necessary as an intermediate stage between the highly-flattened ellipticals and spirals. Later observations (by Hubble himself, among others) showed Hubble's belief to be correct and the S0 class was included in the definitive exposition of the Hubble sequence by Allan Sandage.

Lenticular and spiral galaxies, taken together, are often referred to as disk galaxies.

렌즈형 은하의 대표적인 예로는 M85, M86, NGC 1316, NGC 2787, NGC 5866(Spindle Galaxy), Centaurus A를 들 수 있다.

불규칙 은하

 불규칙 은하 문서를 참고하십시오.

The Large Magellanic Cloud (LMC) - a dwarf irregular galaxy

Galaxies that do not fit into the Hubble sequence, because they have no regular structure (either disk-like or ellipsoidal), are termed irregular galaxies. Hubble defined two classes of irregular galaxy:

• Irr I galaxies have asymmetric profiles and lack a central bulge or obvious spiral structure; instead they contain many individual clusters of young stars
• Irr II galaxies have smoother, asymmetric appearances and are not clearly resolved into individual stars or stellar clusters

In his extension to the Hubble sequence, de Vaucouleurs called the Irr I galaxies 'Magellanic irregulars', after the Magellanic Clouds - two satellites of the Milky Way which Hubble classified as Irr I. The discovery of a faint spiral structure in the Large Magellanic Cloud led de Vaucouleurs to further divide the irregular galaxies into those that, like the LMC, show some evidence for spiral structure (these are given the symbol Sm) and those that have no obvious structure, such as the Small Magellanic Cloud (denoted Im). In the extended Hubble sequence, the Magellanic irregulars are usually placed at the end of the spiral branch of the Hubble tuning fork.

Examples of irregular galaxies: M82, NGC 1427A, Large Magellanic Cloud, Small Magellanic Cloud.

Physical significance

Elliptical and lenticular galaxies are commonly referred to together as “early-type” galaxies, while spirals and irregular galaxies are referred to as “late types”. This nomenclature is the source of the common, but erroneous, belief that the Hubble sequence was intended to reflect a supposed evolutionary sequence, from elliptical galaxies through lenticulars to either barred or regular spirals. In fact, Hubble was clear from the beginning that no such interpretation was implied:

The nomenclature, it is emphasized, refers to position in the sequence, and temporal connotations are made at one's peril. The entire classification is purely empirical and without prejudice to theories of evolution...

The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to be home to many young stars and regions of active star formation, while elliptical galaxies are composed of predominantly old stellar populations. In fact, current evidence suggests the opposite: the early Universe appears to be dominated by spiral and irregular galaxies. In the currently favored picture of galaxy formation, present-day ellipticals formed as a result of mergers between these earlier building blocks. Lenticular galaxies may also be evolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation.

문제점

A common criticism of the Hubble scheme is that the criteria for assigning galaxies to classes are subjective, leading to different observers assigning galaxies to different classes (although experienced observers usually agree to within less than a single Hubble type ). The different classification criteria can also be at odds with each other: for example, a more dominant bulge component does not always go hand-in-hand with more loosely-wound spiral arms. Another criticism of the Hubble classification scheme is that, being based on the appearance of a galaxy in a two-dimensional image, the classes are only indirectly related to the true physical properties of galaxies. In particular, problems arise because of orientation effects (the same galaxy looks very different when viewed edge-on, as opposed to face-on), because visual classifications are less reliable for faint or distant galaxies, and because the appearance of galaxies changes depending on the wavelength of light in which they are observed. Nevertheless, the Hubble sequence is still commonly used in the field of extragalactic astronomy and Hubble types are known to correlate with many physically relevant properties of galaxies, such as luminosities, colours, masses (of stars and gas) and star formation rates.

참고항목

• 에드윈 허블
• 제라드 드 보쿨레르
• Galaxy color-magnitude diagram
• 은하의 형태적 분류

외부 링크

• Galaxies and the Universe - an introduction to galaxy classification
• Near-Infrared Galaxy Morphology Atlas, T.H. Jarrett
• The Spitzer Infrared Nearby Galaxies Survey (SINGS) Hubble Tuning-Fork, SINGS Spitzer Space Telescope Legacy Science Project
• Galaxy Zoo - Galaxy classification participation project.

이 문서는 the politician에 관한 것입니다. Edwin N. Hubbell에 대해서는 the astronomer 문서를 참고하십시오.

Lee Donghyeon
본명	United States
로마자 표기	United States
출생	1889년 11월 20일(1889-11-20) Marshfield, Missouri, U.S.
사망	1953년 9월 28일(1953-09-28) (향년 63세) San Marino, California
안식처	United States
거주지	United States
성별	United States
국적	American
교육	University of Chicago University of Oxford
출신 학교	University of Chicago University of Oxford
주요 업적	Big Bang Hubble's law Redshift Hubble sequence
수상	Bruce Medal 1938
분야	Astronomy
소속	University of Chicago Mount Wilson Observatory
영향을 줌	Allan Sandage

Edwin Hubble (번역 중 : 에드윈 허블)

에드윈 포웰 허블(Edwin Powell Hubble(1889년 11월 20일 ~ 1953년 09월 28일)은 미국의 천문학자로 외부은하를 연구하여 우주 팽창의 실험적 증거를 마련하였다. Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer who profoundly changed our understanding of the universe by demonstrating the existence of galaxies other than our own, the Milky Way. He also discovered that the degree of "Doppler shift" (specifically "redshift") observed in the light spectra from other galaxies increased in proportion to a particular galaxy's distance from Earth. This relationship became known as Hubble's law, and helped establish that the universe is expanding. Hubble has sometimes been incorrectly credited with discovering the Doppler shift in the spectra of galaxies, but this had already been observed earlier by Vesto Slipher, whose data Hubble used.

Biography

에드윈 허블은 1889년Marshfield, Missouri에서 Insurance executive의 아들로 태어나 그 해에 Wheaton, Illinois로 이사했다. 학창시절, 철자법을 제외한 학과 성적이 좋았음에도 불구하고 운동 실력으로 주목을 받았다. 그는 1906년 Single High school track & field meet에서 1등상과 3등상을 도합 7개를 수상했으며 높이뛰기 종목에서 일리노이주 고등학교 육상 신기록도 세웠다. dry-fly fishing도 좋아했으며 아마추어 복싱도 연습했다.

시카고 대학교(University of Chicago)에서 수학, 천문학, 철학을 공부하여 1910년 학사 학위를 받았다. Kappa Sigma Fraternity의 회원이 되었다.(1948년에는 Kappa Sigma "Man of the Year"로도 선정되었다.) 학위 취득 후 3년을 옥스포드 대학교에서 1회 Rhodes 장학생 자격으로 수학했다. 처음에는 Jurisprudence를 전공으로 삼았으나 눈을 돌려 스페인어 석사 학위를 받았다. Some of his acquired British mannerisms and dress stayed with him all his life, occasionally irritating his American colleagues.

귀국해서는 인디애나주 New Albany 고등학교에서 스페인어, 물리학, 수학을 가르쳤으며 남자 농구팀의 감독도 역임했다. Hubble earned admission as a member of the Kentucky bar association, although he reportedly never actually practiced law in Kentucky. Hubble served in the United States Army in World War I, and he quickly advanced to the rank of major. He returned to astronomy at the Yerkes Observatory of the University of Chicago, where he received his Ph.D. in 1917. His dissertation was titled Photographic Investigations of Faint Nebulae.

1919년, 조지 엘러리 헤일에 의해 카네기 재단 산하 윌슨산 천문대의 Staff로 취직하여 사망할 때까지 근무했다. 허블은 제 2차 세게대전 중 Aberdden Proving Ground에서도 활동했고, 이 공로로 Legion of Merit 상을 수여했다. 죽기 직전, 팔로마산의 헤일 망원경이 완성되었을 때 그것을 사용한 첫 천문학자였으며 죽기 전까지 윌슨산과 팔로마산 천문대에서 연구를 계속했다. 허블은 Cerebral Thrombosis(자발적인 뇌 내부의 blood clot)로 인해 1953년 09월 28일 캘리포니아 산 마리노에서 생을 마감한다. No funeral was held for him, and his wife, Grace Hubble, did not reveal the disposition of his body.

학문적 성과

우리 은하 너머의 우주

허블은 1919년 윌슨산 천문대의 100인치 망원경(당시에는 세계에서 가장 컸다.) 이 갓 완공되자 임용되었다. 당시 사람들은 우리 은하가 우주의 전부라고 생각했다. 윌슨산의 Hooker 망원경을 이용하여 안드로메다 은하 (당시에는 성운으로 생각되었다.) 등에 위치한 세페이드 변광성(변광성의 일종으로 나선은하의 표준촉광으로 사용된다.)을 관측하였다. 1922년에서 23년까지 진행된 이 관측에서 허블은 은하들 (당시에는 성운으로 생각되었다.) 이 우리 은하 내부에 있기에는 너무 먼 거리에 있음을 밝혔다. 당시 하버드 대학교의 Harlow 새플리를 비롯한 기성 천문학자들의 견해와 완전히 상충되었던 획기적인 발견은 1925년 01월에 발표되었다.

허블은 또한 현재에도 널리 쓰이는 은하 분류법을 개발하여 허블 연속에 배치하였다. 분류는 은하 사진의 형태를 이용하는 것이었다.

허블의 법칙

윌슨산 천문대의 100인치 후커 망원경

Combining his own measurements of galaxy distances based on Henrietta Swan Leavitt's period-luminosity relationship for Cepheids with Vesto Slipher's measurements of the redshifts associated with the galaxies, Hubble and Milton L. Humason discovered a rough proportionality of the objects' distances with their redshifts. Though there was considerable scatter (now known to be due to peculiar velocities), Hubble and Humason were able to plot a trend line from the 46 galaxies they studied and obtained a value for the Hubble-Humason constant of 500 km/s/Mpc, which is much higher than the currently accepted value due to errors in their distance calibrations. In 1929 Hubble and Humason formulated the empirical Redshift Distance Law of galaxies, nowadays termed simply Hubble's law, which, if the redshift is interpreted as a measure of recession speed, is consistent with the solutions of Einstein’s equations of general relativity for a homogeneous, isotropic expanding space. Although concepts underlying an expanding universe were well understood earlier, this statement by Hubble and Humason led to wider scale acceptance for this view. The law states that the greater the distance between any two galaxies, the greater their relative speed of separation.

This discovery was the first observational support for the Big Bang theory which had been proposed by Georges Lemaître in 1927. The observed velocities of distant galaxies, taken together with the cosmological principle appeared to show that the Universe was expanding in a manner consistent with the Friedmann-Lemaître model of general relativity. In 1931 Hubble wrote a letter to the Dutch cosmologist Willem de Sitter expressing his opinion on the theoretical interpretation of the redshift-distance relation:

[W]e use the term "apparent velocities" in order to emphasize the empirical feature of the correlation. The interpretation, we feel, should be left to you and the very few others who are competent to discuss the matter with authority.

Today, the "apparent velocities" in question are understood as an increase in proper distance that occurs due to the expansion of space. Light traveling through stretching space will experience a Hubble-type redshift, a mechanism different from the Doppler effect (although the two mechanisms become equivalent descriptions related by a coordinate transformation for nearby galaxies).

In the 1930s Hubble was involved in determining the distribution of galaxies and spatial curvature. These data seemed to indicate that the universe was flat and homogeneous, but there was a deviation from flatness at large redshifts. According to Allan Sandage,

Hubble believed that his count data gave a more reasonable result concerning spatial curvature if the redshift correction was made assuming no recession. To the very end of his writings he maintained this position, favouring (or at the very least keeping open) the model where no true expansion exists, and therefore that the redshift "represents a hitherto unrecognized principle of nature."

There were methodological problems with Hubble's survey technique that showed a deviation from flatness at large redshifts. In particular the technique did not account for changes in luminosity of galaxies due to galaxy evolution.

Earlier, in 1917, Albert Einstein had found that his newly developed theory of general relativity indicated that the universe must be either expanding or contracting. Unable to believe what his own equations were telling him, Einstein introduced a cosmological constant (a "fudge factor") to the equations to avoid this "problem". When Einstein heard of Hubble's discovery, he said that changing his equations was "the biggest blunder of [his] life".

다른 발견

1935년 08월 30일 허블은 소행성 1373 신시내티를 발견하였다.

저서

1935년 경에 저술한"우주론의 관측적 접근" 과 "성운의 Realm" 등이 있다.

Nobel Prize

Hubble spent much of the later part of his career attempting to have astronomy considered an area of physics, instead of being its own science. He did this largely so that astronomers  — including himself  — could be recognized by the Nobel Prize Committee for their valuable contributions to astrophysics. This campaign was unsuccessful in Hubble's lifetime, but shortly after his death the Nobel Prize Committee decided that astronomical work would be eligible for the physics prize.

On March 6, 2008, the United States Postal Service released a 41 cent stamp honoring Hubble on a sheet titled "American Scientists." His citation reads: "Often called a 'pioneer of the distant stars,' astronomer Edwin Hubble (1889-1953) played a pivotal role in deciphering the vast and complex nature of the universe. His meticulous studies of spiral nebulae proved the existence of galaxies other than our own Milky Way. Had he not died suddenly in 1953, Hubble would have won that year's Nobel Prize in Physics." The other scientists on the "American Scientists" sheet include Gerty Cori, biochemist; Linus Pauling, chemist; and John Bardeen, physicist.

영예

수상

• 1938년 Bruce Medal
• 1939년 Franklin Institute Awards
• 1940년 왕립천문학회의 금메달
• 1946년 Ballistics 연구에 크게 기여한 공로로 Legion of Merit 수상

사후 그의 이름이 붙은 것들

• 소행성 2069 허블
• 달의 허블 분화구
• 허블 우주 망원경

• 뉴욕 주 브루클린에 위치한 에드워드 R. 머로우 고등학교 천체투영관]
• Edwin Hubble Highway, the stretch of Interstate 44 passing through his birthplace of Marshfield, Missouri
• The Edwin P. Hubble Medal of Initiative is awarded annually by the city of Marshfield, Missouri — Hubble's birthplace
• Wheaton high school이 1992년 허블 중학교로 전환됨.
• 2008년 미국 우표 "미국의 과학자 시리즈", 액면 $0.41

See also

• Astronomy
  • Distance measures
  • Cosmic distance ladder
• Galaxies
  • Hubble sequence
  • Galaxy morphological classification
  • Gerard de Vaucouleurs
  • William Wilson Morgan
• Expansion of the universe
  • Big bang
  • Albert Einstein
  • General relativity
  • Hubble's law
  • Hubble constant
• Hubble Space Telescope
• Edwin Hubble House, residence and National Historic Landmark in San Marino, California
• The Great Debate of 26 April 1920

Further reading

• Christianson, Gale; Edwin Hubble: Mariner of the Nebulae Farrar Straus & Giroux (T) (New York, August 1995.)
• Hubble E.P., The Observational Approach to Cosmology (Oxford, 1937.)
• Hubble E.P., The Realm of the Nebulae (New Haven, 1936.)
• Hubble, Edwin (1929). “A relation between distance and radial velocity among extra-galactic nebulae”. 《PNAS》 15 (3): 168–173. doi:10.1073/pnas.15.3.168. 
• Mayall, N.U., EDWIN POWELL HUBBLE BIOGRAPHICAL MEMOIRS NAS 41
• Osterbrock, Donald E.; Joel A. Gwinn and Ronald S. Brashear (1993년 7월). “Edwin Hubble and the Expanding Universe”. 《Scientific American》 269 (1): 84–89. doi:10.1038/scientificamerican0793-84. 
• Harry Nussbaumer and Lydia Bieri, Discovering the expanding universe. Cambridge University Press, 2009.

External links

• Time Profile
• Astronomy at the University of Louisville — Photographs of Edwin Hubble at New Albany High School.
• Edwin Hubble bio — Written by Allan Sandage
• O’Connor, John J.; Robertson, Edmund F. “Lee Donghyeon”. 《MacTutor History of Mathematics Archive》 (영어). 세인트앤드루스 대학교. 
• American Physical Society's Hubble Bio
• Edwin Powell Hubble — The man who discovered the cosmos
• Astroprof's article on Hubble
• Hubble: The Man and His Telescope — slideshow by Life magazine