PHONETICS: /ɑ:/

In British English, the phonetic symbol /ɑ:/ corresponds to the vowel sound in words like "art" and "ask". It is called a long vowel. 

Some common words which practice the pronunciation of /ɑ:/ include the following:

• with "a": after - ask - bath - half - staff - task;
• with "ar": are - aren't - arm - art - bar - car - dark - farm - hard - harm - part - smart - star - start;
• others: aunt; heart; laugh;
• homophones: aren't/aunt

Many Spanish speakers have difficulty remembering that /ɑ:/ is pronounced the same whether there is an "r" after the "a" or not.